I haven't used TradeHill, but it looks like they take a much bigger cut at least with instant buying and selling...
TradeHill Buy: 20.12
TradeHill Sell: 17.64
MtGox Buy: 17.51
MtGox Sell: 17.50

$2.48 vs. $0.01 difference, seems like a obvious choice.  You should take note that MtGox takes a 0.65% fee, so to actually buy 1 BTC it would be $17.62 and to sell 1 BTC it would be $17.39.  So in reality the difference between MtGox's buy and sell is currently $0.23 vs TradeHill's $2.48.  I'd recommend MtGox with Dwolla.  Dwolla is simply enough to use and add a bank account to.