you should really search this one, as it's been discussed probably more than anything else on this board. anyway, there are very few differences beyond aesthetics...some will argue that brands such as K&N, AEM, Injen have spent so much $ on r&d that they perform better; others will say that's bs. I have the ebay CAI, as do many others, and am fine with it personally. Save your money, IMHO, and buy it. There's no reason to spend upwards of $150-200 for a very, very small power "increase".
again, though, search "ebay cai" and read up so you can make an informed decision. oh yeah, welcome to the forums.