If your shooting core lokt's or such, the 870 is going to a) give you more barrel to burn powder, and generate more velocity,
b) soak up a hell of alot more recoil based on weight. easily 1.5 lbs heavier.
(nova's are light as hell which is a plus minus depending on your opinion)
and c) the cost of extra parts barrels stocks etc. Im not a big 870 guy, but you can get ANYTHING for them for relatively cheap.
And one more plus for the 870, the R3 is one hell of a pad, and soaks up recoil well.
Parts and barrels are rather limited for the nova.
weight wise, i would compare the nova to my winchester 2200.
I would take the recoil of my 14 inch 870 I had over the recoil of my winchester 2200 with turkey loads. Even though the action of the winchester soaks up the recoil a bit, at just over (and I mean fully loaded to be over) 7lbs, it kicks the #### out of you.
And you can compare a core lokt to a 2 oz turkey load IMHO
Its Ok to shoot shot through a rifled barrel, but like any other sabots (which is basically what your shot cup is) it will foul your barrel, and your pattern wont be #### hot at all. But for a grouse @ 15 yards, sure as #### its dead.
Apparently Stoeger has a copy of the Nova available for much cheaper.
Might be worth a look too...