That depends on your priorities.
If you want to play online with your brother, you would need a 360 (obviously)
If you still want to play with you brother, but playing splitscreen isn't a problem for you, I would get the PS3 (because you can play 360 splitscreen games on his 360 and vice versa with your PS3)
Of course, if there are a lot of games you want to play by yourself/online with other people on the 360, then you should put those games up against the selection you have on the PS3. (Unless you can get a lot of alone playtime from your brother on his 360)
Oh, and when you say Ratchet & Clank and Uncharted, you do realize that both have sequels in the works, right?