If asking for opinions and objective experience, I will offer my perspective as I have extensive experience with the SBE 3 and I have handled and shot the older A400 Xtreme and newest A400 Xtreme Plus.
Both the SBE 3 and A400 Xtreme Plus have their pros and cons, but of any of these I have never witnessed personally or heard from a credible source of reliability and durability being anything but exceptional for both models. When it comes down to it, with proper full power hunting ammunition both shotguns will chug through quite a bit of dirt, grime, cold, wet and everything else Waterfowl hunters will throw at their shotguns. So from that angle, there is not a bad choice and you really need to pick which model fits you best. Trigger time with both will absolutely help you in your decision making, if you have that opportunity.
So, with that said, I'll address the elephant. For what it's worth, I like both guns but I am an SBE 3 owner and shooter (liked it so much I bought it twice, 12ga Supermag and the new 28ga), so I do have an inherent bias, but I will be as objective as possible. (The too long, didn't read synapses of my evaluation is I have two, one shot high, I adjusted it to pattern the way I want and I have never had a single problem with any other aspect of the shotgun.)
All reports of the SBE 3 suffering any sort of problems have been related to pattern splits and zero, rarely reliability, durability, or ergonomics. If you go back and look at it, the same sort of issues were present in the SBE II when they transitioned to the higher vent rib profile and modified the design slightly from the SBE. Was it as egregious as what has been accounted for in the SBE 3? Not in general, but it was there. After the first few years the voices quelled and you would still hear the odd owner or two have some sort of a pattern split issue, but it was not considered a "design flaw". There is no doubt that the 3rd generation of the SBE has suffered from excessive pattern height issues in multiple production runs. Some of this is Benelli's fault and some of it is the failure of expectations of your average American hunter who always seem to think that you have to have a 50/50 shooting gun to hit a bird and you have to "aim" a shotgun; plus, if someone has something they can grab onto and use to blame the bow and not the indian, they will do it for their own mental gymnastic benefit. I'm not saying that this is the case with everyone who has had a problem, but I have heard a boatload of guys say "mine shoots high too!" online. When asked what they did to pattern, how they confirmed this, the sight picture they are using, does the gun fit them, etc, there is a notable number of them that fall back on the "well, I basically shot a insert target that is not a pattern board and a gun should shoot where you aim it anyways, so I FEEL it shoots high" line of reasoning. So, it's been hard for me to really determine if Benelli fixed the high patterning issue post Q2 2018, or if the estimations of over 40 percent of the SBE3 12 and 20ga models shooting "high" is factual. It was certainly, and maybe still is, an issue for many of the first year and a half of production, and the 20ga model had reports of the same post release. Benelli had a massive PR failure in telling the market in that in response to the criticism and real issues customers had, they said "here's our loose specs on pattern splits, just shim the gun you idiots!", which went over about as well as a dead fish in the punch bowl. They really needed to provide more information and be transparent with their customer base on what issues were actually present and how they were remedying them. Instead, their corporate decided to take egg to the face and be too proud to admit there were some ghosts in the machine that needed to be worked out.
That said, what you don't hear much about is the number of owners that have been very happy with the patterning splits of the shotgun or how many examples are shooting in the 60/40 to 70/30 spec. The fact of the matter is the gun is designed and intended to be tuned for a 60/40 to 70/30 pattern split and is equipped with a rib that places POA/POI intersection at about 16 yards. If the gun has not had the Barrel Stop Ring set properly or it simply suffers from tension bringing the barrel upwards (anyone's guess as to why), then it will inevitably shoot an 80/20 to 90/10 pattern split, with some going 100/0 on the really bad examples. This issue compounds when you have many who do not know how to shoot target focused and "aim" their guns, or have bad mounting practices/have not adjusted drop and cast to bring their eye in line with the bore. So, from a manufacturing/engineering and marketing standpoint a company needs to either decide to accommodate to the lowest common denominator of the masses or create the product(s) they think is best for the market and preach/educate on how the product is intended to work and best practices for better results. Benelli has, for the most part, chosen to just throw their product into the wild and say "it's the Best!" and not tell you why or how it's "the Best" in function. Which, even if it is, will make many just disregard it or hate on it. They are doing something about it though, as I have noticed recent examples on gunstore shelves no longer have a mid-bead. To me, this was always a nice extra on a gun that can be easily removed if the shooter does not like or need it, but it does cause some shooters to assume that they need to stack the beads for a "sight picture" and inevitably send the pattern sailing high as they look over the barrel and usually hold the bead dead on our just a touch bellow target center. This is not conjecture, I have witnessed this happen with my own SBE 3 example when handing it off for others to try. Quick correction, and suddenly the user starts crushing targets. So, maybe Benelli has decided to accommodate for the market and do something to help end users?
Whatever the case may be and the arguments for or against the SBE 3, the bad press has not disuaded ample numbers of folks from buying the gun like hotcakes and encouraged Benelli to release it in 4 different chamberings (3.5" 12ga, 3" 12ga. 3" 20ga, 3" 28ga) with multiple barrel lengths and finishes. If it were that bad, would so many hunters keep buying it? Maybe, but most likely not...
Now, the A400 Xtreme Plus is a damn good shotgun, but I have personally not seen too many out in the wild compared to other high dollar duck guns. It is a little heavy compared to the SBE 3 and others, and it has some slightly odd looks. The spring loaded stock takes some getting used to and the action is inset into the reciever with an odd zipper sound when cycled. This has nothing to do with performance, but looks and feel do play a part in how well a shotgun sells. Waterfowlers preach practicality and effectiveness all day long, but really we make a lot of choices based on aesthetics and "cool factor". In my limited shooting experiences with the older Xtreme and newer Xtreme Plus, I found them to be very well pointing and solid feeling shotguns. They felt chunkier than they actually weighed though, and it was slightly odd coming from inertia and old duck guns to that, but it was not a bad experience at all. I crushed clays and patterns at a glance looked good on the board with both fine shot and buck. It does have a mid-bead, which some see fit to remove, and I do know that with some loads it will pattern a little high (60/40-ish), which is not a big deal and is very example dependent.
Reliability was perfect and the long forcing cone combined with over-bore pretty much mean you will find very few quality loads that the guns will not like to pattern well. I like the choke design, and there is plenty of aftermarket support for the model. At the end of the day, it is not my pick, but if you gave one to me and said I had to hunt for a season with it and nothing else, I wouldn't really feel cheated, but I'd have my gripes. I don't like the plastic bolt handle nor the cartridge drop lever design, so those features are kind of dinky to me. The cartridge drop lever is definitely not a great design for gloved or cold-numb hands, but for many it's not a big enough of a worry to dissuade them from buying. The enhanced bolt release is fantastic and really gives you plenty of surface area for quick and efficient reloads in the dark or when not looking. What apears to be a soft comb piece at a glance much like Benelli's Comb tech in their gen 3 Comfortech stock, is actually just a big black plastic section of the stock. It does nothing to soften the blow to your face, and while your shoulder is protected, if you are off on your mount with heavy loads I found I would get slapped in the face a little. Heavy loads will not be pleasant to a cold face on a rushed mount in the field, but that may be getting picky. The comb is wide enough it doesn't feel like it's trying to cut you when you're slapped, but it's still a punch. Grip fills the hand well, but if you like more daintily wristed shotguns, this is not the smooth bore for you.
All in all, I really think it comes down to three factors for this choice:
1) Which gun fits you best and handles the best for you?
2) Which feature set makes you feel like you are getting the best deal to fill your needs?
3) What gun's potential shortcomings are you most willing to potentially live with or feel confident in addressing should they be an issue for you?
Really, if you get a 2/3 for one or the other their is your top pick and the right gun for you. Hope this long winded diatribe of information helps!