Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Not sure what your talking about by browser, I'm assuming you mean the skins you can install. I have a jailbroken ATV2 with XBMC I setup for my parents that I mess around with whenever I'm home. From a speed perspective Quartz is probably one of the quicker skins as there isn't much to it and its styled to look like the Apple ATV2 interface. The AEON skins tend to be too processor intensive to work very well on the ATV. I would just check out the skins and install a couple and mess with them till you find what you like. I personally like Neon and Cirrus Extended, both work decently well with a bit of pause during the transition between menus.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.