Well I've got a 2.7 so of course I voted for that
I know someone with a Boxster S who says that the 2.7 base handles better than the S due to it's lighter weight.
Boxster 2.5 = 1252 Kg
Boxster 2.7 = 1260 Kg
Boxster S = 1320 Kg
In the S compared to the 2.7, you may have more power, but you've also got your girlfriend in the car with you, which is really not going to help with fast cornering
I'd say the 2.7 base may be the best compromise, unless you are into drag racing. However, I've never driven an S, and I should probably avoid trying one just in case
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