I don't think I've ever used a Roku stick, I think the only Rokus I've ever used were boxes. The interface is probably the same, I'd guess.
I've used Firesticks for years. If you're a Prime Video user, the Firestick might be a tad bit more convenient in its home screen setup. Overall, I don't think there's much difference between them. The Roku feels less pushy and intrusive.....it feels a little bare bones on the home screen, which is likely preferable for many people. Doesn't feel like it's trying to push anything on you. The Firestick home screen advertises movies, tv shows, apps, suggests a lot of things to watch.....I happen to like that because it's how I find out about some things, but if I wanted to be left alone and have a more stripped-down experience that shuts up and just lets me get to my apps, I'd probably go Roku.
my $0.02