Head on down to the club and check out what everyone is running. You will be able to get much more help on setups and parts if you are racing the popular choice.

As to which is better: if you pick up a buggy from any of the major manufacturers (Asscoaited, Losi, or Kyosho) you will have a potential race winner. None are significantly faster than the other, unless you are right at the pointy end.

My vote: Associated all the way.