I play on both those realms, mostly on Outland nowadays since I switched main. Outland is a really nice server for pvp, good population on both factions, though alliance is a bit larger. In tradechat you always see people announcing for RBG's and arenas both low rating and high rating. It can be 10 min queues in the evening.
I have only played horde on Ragnaros, and back in cata it was a nice pvp server, even though horde didn't have much RBG's, but I think alliance did. Don't really how it is now. The cool thing with Ragnaros is the equal population between alliance and horde, 50/50.