If you do decided to go with the Mustang, wait until next year. It's getting an engine upgrade to 350Hp +, and it's not just a tune and a few easily replaced items. IIRC they're actually upping the displacement a little, and it's getting revised heads.
The 335i is a great option, but if you're going to be streatching the budget to get it, the Mustang might be a smarter move. There's plenty of aftermarket support for either, but the Mustang is going to cost you less if you break something.