I have not used either yet but I think MtGox is the biggest and Tradehill is a lot newer.
I do like the way Tradehill displays the current prices over MtGox, much easier to read.
There is also
https://www.bitcoinmarket.com/ but they are invite only at the moment.
Agreed, when I started out last friday I registered with all the "main" sites that I read about. I didn't even bother with MtGox because of its interface.
Trade Hill Looks alot more professional. I've traded about $60 worth of BTC so far using it.