There are applications that will use your videocard for encoding video actually. I believe Avivo is built into CCC but I've never used it. There's other software that will let you use Nvidia cards. In terms of how good those 2 cards are at it I really don't know but it likely depends on the software. In terms of gaming they are fairly close so I'd think that would roughly apply to their encoding abilities as well.
I've tried Avivo with the old Catalyst 9.7, and I wasn't impressed by the quality. It may have done things in 15 mins as compared to about an hour on a cpu, but the quality on cpu based encoders is still better (though it takes time). I don't know about the latest version of CCC though.
On the nvidia side of things Badaboom is the leading converter, though it isn't free and the options you could change are a bit limited.