Go test drive the 987 S and let us know of your impressions. My guess is once you taste the mid-engine Porsche experience you will never want to go back to anything else. I did test drive your 3.0 Z4, the M3, the Audi S4 -- and have driven a lot of other sports cars. I clearly remember the day when I test drove the base 987, a 911, and the 987 S, in that order in the same day. I knew right then and there that I'll be getting a 987 S. This car changed the whole concept of driving for me. I never knew that it could be so much fun and that I'd be doing so much driving just for the fun of it -- which is pretty much 100% of the time in the Box and on weekends, since I don't commute by car.
In value terms, IMHO, a bare bones (or a very slightly optioned) Box S (new in the low $50k area with discounts) is the best value in the sports car and roadster universe. Once up there, I wouldn't let a few $k make the decision for me, especially since in a Porsche you are paying for many things that are deeper than mere hp and acceleration numbers. There's a lot of hidden technology under that skin that undeniably produces that Porsche feeling, and to me, that's worth every penny in price difference, and more. But hurry before Porsche discontinues the Boxster and introduces the Cayman Convertible for $10k more.
Only problem for me now is that I'm going crazy looking at the impending FOURTH nice weather (50+ deg) weekend AFTER I put my box in winter storage.
I honestly don't know how I'm going to control myself if the first snow storm doesn't come soon.
Z.