Can I cop out and say that both genres are good?

I like FPS games for quick sessions - I'll do a mission offline in GRAW if I have an hour or so to play. If I only have 20 minutes, I'll play a quick deathmatch in Unreal or whatever.

When I've got most of a day free, I'll play either Neverwinter Nights, or an MMO.

I don't think the genres could replace each other - they're both fun, but you can't get engrossed in an RPG in a short play session IMO.