MtGox does allow you to withdraw in USD, it's just that you get put on a long processing queue. The problem is that their Japanese bank has restricted them to only 10 international wires per day. They've been having a really tough time with banks. Interestingly, if you have a Japanese bank account, you can withdraw Yen from MtGox within a few hours.

Regarding price - MtGox's has been fairly in step with BTCChina's price. BTCChina and MtGox both represent the top exchanges by 30-day volume, so it would seem to me that MtGox's price is more accurate than BitStamp's price which lags miserably behind every rally.