[Perfectlap]

If you are asking about buying a 986 S (3.2L) vs. 987 (2.7L)
I would say don't buy a 987 they have ugly headlights.

seriously, it would depend on how much you can get the 986S for.
If its not a serious mark down I would go for the newer car. I would look at
the likely depreciation of the two. My take is that you take less of a hit with a very popular used car. The pool of buyers is inevitably larger for people looking to spend 25-35K than those looking to spend 35-45K.
I got my 2000S for 30K with 10,000 miles last fall. Its my hope that it will be quiet some time before it get down to $20K. Historically I've never put more than 10K miles a year on my cars so hopefully that will help out over the next four years.

From a performance standpoint the power of the new Boxster is close to the old S model so you won't be making a huge swing either way in that department.
And I strongly feel that neither the S or the standard are underpowered.
Take both to a Porsche driving school and when you set the absolute quickets lap possible in either, then tell me you need more power. The Boxster is sportscar not a drag strip car, there is more than enough power to get you into trouble.
A friend of mine used to race junior formulas with a current f1 driver and he recently told me that this professional driver sent his father a new Mercedez compliments of the team. After driving it he STRONGLY noted that its 500+HP should not be sold to the general public. Even if you are only intending to drive in straight lines you need to master the power you have before you complain about not having enough ooomph.