Dang I like that shop... $100 for a used single shot and selling 835s for $165!

Its completely up to you. You don't have to shoot 3.5" out of an 835, but I will say that those over-bored barrels throw some pretty nice patterns. It also gives you the ability to shoot any 12ga shell... so if you ever did want 3.5" for any reason, you have the vehical to do so.

Handle them both a bit. Which ever is more comfortable to you, will work out better. It needs to fit right first and foremost.. I still kill lots of ducks with a 20ga every year... so as Colo. said.. you don't need a roman candle to kill a duck.