The new (2013) Boxster S is... well it's incredible. The redesigned exterior (IMO) is fantastic. The top of the car is a lot more flat than it was in years past and the front end is a little more squared off. I'd always thought that they were too rounded for my tastes, but the new one looks like a damn supercar. It drives like its on a rail, daring you to push it harder into turns.
Unfortunately, they're also more expensive and you're unlikely to see the deals that you will from BMW.
I love the Z4. They're (IMO) more pleasant inside, particularly with the added tech (comfort access, iDrive, etc). Spec'd out, they are nearly identical from a performance perspective. Hard to beat the hard top.