There shouldnt really be a performance difference if both are run using the same settings.
The difference is that if you buy generic, you might end up with PC-133 that wont do any more, and only at CAS3, while if you buy some fancy RAM, you'll know what you get.
Personally Im using generic cheapo RAM myself, cause it feels like such a waste to buy new RAM when the RAM I have works fine, though only at 100 MHz CAS3.