I still prefer BotRK to Hydra. Specifically, we're probably comparing actives. Hydra gives you around 100-90% of your AD for the active, whereas BotRK gives you 150 + 50%. That makes BotRK better until you get 300 AD (which I'd say is pretty late in the game), some movespeed, and it gives you that passive which is super-useful in the League of Warmog's.
They're both pretty much similar, though. AoE vs. single target, I guess. I just generally don't get Hydra unless I'm against a super-pusher like Singed or Rumble where I want to counter-push.