They're really such different cars that it's hard to compare them. The Boxter is a mid-engine convertible sports car, the BMW a front-engine, rear-wheel-drive sports coupe.
I'd take the Boxter, unless you think you'll need the back seats, or unless you live in a part of the country with a lot of cold weather and/or snow.
I can't seen to decide between the porsche boxster or the new 2007 bmw 330. I want a fun car that will last. Can u guys help