Like everyone's been saying, for $30 go with the 3.5" for a turkey gun. I bought a dedicated turkey gun this year as well and my decision came down to the same 870 you're looking at and the 835 ulti-mag for my price point. Really can't go wrong with either, but I decided to go with the 835 cause it was better suited for turkey hunting right out of the box, had a longer barrel (24"), was heavier (and my shoulder thanks me) , and it always seems like when pitted against one another the 835's will pattern better (not sure if it's the overbored barrel or what?). Whichever way you go, make sure to get it soon enough so you have some time to pattern some different loads before you hit the woods. Buying a new gun is like Christmas ;D
PS: Keep an eye out on armslist, there are some deals to be had. Wound up getting a steal on a brand spanking new ulti-mag for $350, then fitted it with a Burris Fastfire III (my perfect turkey gun for about the same price as retail).