Lets be honest these are both going to be cannabilised by GTA 5. That other small open world game, the one that doesn't release annually and usually throws the industry into disarray after release.
I am planning on getting both, actually they are the only games I'm planning on day one next gen.
I still think ubisoft should have spun the ship/pirate stuff along with the levelling system in brotherhood and ac3 for the crew of the ship instead of assassins off into its own franchise a la far cry/blood dragon.
I'd be far more willing to throw money at a product that didn't contain Desmond, or any of the ac story shit, I mean seriously ac is stupid as fuck.
I love old naval ships! One of my favorite series of books are the Patrick O'Brian series of Napoleonic age Aubrey–Maturin nautical historical novels. They are very historically accurate, funny, and great adventures too.
I wish the Ubisoft Assassins Creed engine could be used to recreate those books. But in the meantime we have this game which will have to do.
On the other hand, if you like what you see in this game read "Master and Commander", not the novelization of the movie, but the original book. Or, better yet get your hands on the audiobook which is excellent and has all the voices in various English, Irish and Scottish accents.