Brotherhood is "better" with a better story (ever so slightly) and better sidequests, better assassin mission minigame...
Revelations has less glitches/bugs, better action scenes, better 100% sync optional objectives. And a better city (in that it feels less dull), as well as more features. It also has better multiplayer, and i assume it's much more active.
I'd say you'll want to get both. Especially if you're following the series. Both games have bad pacing and feels like a shallow quicksand box (there's lots to do, no order to it, you'll be stuck quickly renovating and working on challenges and sidequests, but there's not really that much to do of any one thing, very little depth to the things you do, and they both feel like they're over too quickly).
Short version is: Revelations is better but shorter and with less engaging sidequests. Brotherhood is not as good, but longer due to it's heaps and heaps of sidequests.
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Both games should have been released as DLC's.... don't think it's possible? Look at Episodes from Liberty City.
Brotherhood was originally going to be a part of AC2 (the AC2 dev diaries have them saying it ends in 1503), but then they decided Rome needed its own game. Luckily, they added a LOT of content with Brotherhood.
Revelations was not supposed to exist. The devs said they weren't going to make a game for release in 2011, but rather have a second larger DLC for Brotherhood. When the 3DS was announced, one of the titles was Assassin's Creed: Lost Legacy. It was about Ezio going to Constantinople and searching for Keys to Alta´r's library. Sound familiar? A few days after the devs said there wouldn't be an AC title in 2011, the executives said YES, there will be. Lost Legacy was quietly canceled, and Revelations was born.