Well in that case if your 870 is your main gun and if you prefer Winchester over Remington I would hop on the deal as then it would be your primary gun but... I know this isn't true with you having two semi auto duck guns it would likely be your third line of defense if the guns go down. If you were to hunt in some pretty crappy conditions like in Canada or even MN,ND, north SD, WI, etc. your northern duck states you may want to use your Vinci over the Urika just because of the fact that it could freeze which is very rare but can happen and have you sxp as your backup or even use it as your primary just so then you know its not gunna F up on you. in my opinion back to the pumps the sxp is the nicest shot gun in the price range it is. I would say in the $400 or so price range it would go Winchester SXP,Benelli Nova, Weatherby P08, Mossberg 500 835 535, Remington 870. The best pumps on the market right now are the Browning BPS and Ithica m37 Which I would take the Ithica over the browning. Now back to your thinking about getting the SXP waterfowl 3.5" I THINK it is a great price considering it is $50 off from my local gun store and it is a very versatile gun because of the light weight meaning you could use it for literally anything be that dove, goose, duck, pheasant, coyote, clays, deer, hogs, The list goes on. The other nice thing about the gun is that you can buy a slug barrel for it if you live in a slug only district or state and if you want to hunt deer you can get a slug barrel for a lot cheaper than your Urika if they even make one... But, the slug barrel will cost more most likely than if you were to get a slug barrel for your 870. Honestly though it is up to you. For me I would hop on it like once again it is $50 off not much but can help. Good luck. Also the sxp SHOULD pattern better than your existing guns especially with big loads such as #3,#2,#1,B,BB,BBB,F,T. Or with heavy Turkey loads.
Winchester SX3 3.5" 28" 12ga MOSGB
Benelli Montefeltro 26" 12ga
Remington 700 sps Varmint .223 Rem
Remington 597 .22lr
Marlin Model 80 .22lr
Izhmash Mosin Nagant 1942 7.62x54r
J Stevens .22lr Single Shot Pistol
SHOOT EM' UP!