Both are good guns, But .
I like the 870 Supermag.
A proven design .
Lots of extra barrels out there ( extra 3 1/2 and the 2 3/4 and 3" fit just have to use the right shell).
Lots of scope mounts ,stocks, extended mag tubes ect.
If you are tall (or grow) there are long stocks , if you or a youth need a short stock a replacement short stock is easy to be had,or cut down .
The 870 design is durable, I have a 26" syn stocked Express supermag , It has been drug thru the (and droped in the )mud ,dunked in the marsh and I kept shooting.
I used it as a push pole and pattle when I broke the cannoe pattle.

Yes it is scratched and mared but it has not failed me ,I bought it the first year Rem released them.
Heven only knows how many shells I have fired thru it.

Also the Nova's stock can't be replaced if you would ever want to or if it dosn't fit you( grip to trigger is wrong for me).

Johnch