The 3.5's advantages are mainly when it comes to waterfowl, since there's more shot, it's better when shooting at passing waterfowl.

With Turkeys, it doesn't make such a huge difference as far as killing them is concerned, I myself shot my first turkey with a 3" in the chamber, but I honestly would still prefer the 3.5", when you look at the heads of the turkey I shot with the 3", and the other one my buddy killed with a 3.5", trust me, there's a difference, no matter what people say about the whole "dead is dead, can't get no more than that" argument, there is a difference.

Get the 3.5"