Revelations is the Assassin's Creed game you should only get if you're genuinely interested in how Ezio and Altair's stories close, because that's all it really does. There's little to no storyline progress (the Desmond sections all take place in the Animus for Christ's sake), and other than taking place in Constantinople instead of Rome, it's not really different from Brotherhood.
I really enjoyed it, don't take me wrong, would play again, but I'm also really glad I got it for 30 bucks.
On the flipside, I only played the start of Arkham City. Didn't really care for it, but I can't pass judgement, it was Christmas morning and I was waiting for shit to happen. When it did I had Super Mario 3D Land to play so I haven't touched Arkham City since.