Unless you shoot geese, and do it often enough to buy ammo by the case, you have no need for 3.5" shells. They do help if you have to shoot heavy loads of large steel shot. For everything else a 2 3/4" or 3" shell is just as good, if not better and will not beat you up too bad. For the occasional goose hunter I'd use 3" shells and just buy the more expensive Bismuth or Hevi shot. This shot is actually better than the 3.5" steel, but too expensive if you plan on shooting it by the case.
As far as the Nova VS the 870, I actually like both guns. Probably because I'm more used to the 870 I shoot it much better. But I still like the Nova a lot. If I were just starting as a shooter and had no real experience with either I could see me going with the Nova. I've had a couple of the Nova's, really want to like them, but always went back to the lighter, better handling 870.