Well I have to disagree with the 3.5 inch statement, plenty of ducks and geese are killed with a 2 3/4 or 3 inch steel shell. I personally don't see much of an advantage in the 3.5 inch shell, it only allows you to put more shot in the air (which isnt a bad thing), but they do kick a lot more and may not pattern as good as the shorter shells.
BUT... none of what I said matters because both the nova and the supernova shoot 3.5 inch shells, so you can shoot any shell you want. Personally I would go with the supernova if you can save up the extra cash, the comfortec stock that is comes with (basically the only difference between the two guns) does reduce some recoil from what I have seen on my M2. And from the supernovas that I have seen, most dont have the rattle in the forearm like all of the novas do.
So if it were me, go with the supernova, its probably one of the best pump guns out right now.