First off let me say that both are excellent for waterfowl guns.....or for whatever you intend to use it for
That being said, I'll vote for the M2 Field everyday. I was in the same predicament as you a few years ago. I went ahead with the M2 and have never look back. This is why:
1) Gun fit: Perhaps the most important thing when looking at a gun intended for wing shooting. Both SBE2 and M2 should great and point/shoot right where I'm looking. However, the M2 is a tad bit lighter and balances better IMO. My M2 fits me like a glove, where as the SBE2 felt slightly less comfortable. I would suggest comparing the two guns side by side in the gun shop, then do the same thing with the same model but comparing 26" vs. 28" barrel. Personally, I like the "closed eye, shoulder and swing" method. Without being told which gun youre being handed, close your eyes and shoulder the gun like you would in the duck blind. Swing back and forth (gently). Make a desicion based on feel and which one shoulders most comfortably for you! A gun that you can shoulder and fire w/o much thought or fudging around will no doubt be the gun that you shoot best with in the field.
2) 3" vs. 3 1/2": Guys will never agree on this subject; whether to have the 3 1/2" capability and not use it or supposedly further range to hit passing ducks (don't believe this thought, if you need 3 1/2" shell over a 3" shell to hit a duck you should seriously reconsider your M.O.). I use my gun for everything from ducks and big honkers to any upland applications I need (turkeys included). The M2 even does very well on the sporting clays course....I've put in a couple summers of hard clay shooting with my M2, its great!
If you can refrain from busting at birds at 50+ yds and can decoy them in, to make a well placed and accurate shot then only have the 3" capability is not a problem. Besides, 3 1/2" shells cost a heck'ova lot more too!
I have not once felt like I was undergunned with my 3" chambered M2 Field.
3) Reliability: More reliable/issue free than SBE2?
M2's are really the workhorse of the Benelli line up. Throw this gun at any application and it performs like a champ. I've refrained from cleaning my gun for months at a time in an effort to make the gun stop functioning....it would not. Never have I had an issue to cycle in the 3 hunting seasons I've had mine. Been through some nasty conditions and keeps on going "Bang". I added a Wolff's 25% stronger recoil spring to my gun, I cannot describe how great it has been in improving the speed/locking strength of the bolt. I recommend this spring as it is an easy install and only $12.99 last time I checked.
I've killed lots of birds with this gun, been lights out...perfect. You won't go wrong with the M2 Field, and you won't go wrong with the SBE2 either. As I mentioned, pick the gun that simply fits/feels/shoulders the best and you'll have the best weapon FOR YOU!