Boxster S Pros: Loud, handling, storage space (Its got 2 boots :o) , over-engineered cupholders, looks like a mini-911
Cons: New gen around the corner, soft-top, unimpressive interior, looks like a mini-911, expensive to maintain
Z4 35i Pros: Turbo surge, interiors, features, overall quality, looks, better to mod, BMW reliability, its a coupe, its a convertible, DCT
Cons: No cupholders (RHD), handling (feels like understeer as we're too far from the front), HPFP scare (never had one go bad), could do with more storage space
I put down my money on the Boxster but then switched to the Z4, no regrets.