My 2 cents for what its worth is go for an S if you can afford it. Have a Porsce Pre purchase inspection completed at a dealer.
As mentioned above HP an S version has other upgrades over a base. Also I have never heard anyone say they had toomuch horse power let me reduce it.
In the end you should test drive both and make your decision, and be happy with it. A boxster drives awsome, handles like no other car i have driven.