I too had this dilemma before buying my Boxster S last spring. For pure performance the ZO6 kills the Boxster. Upgrade and maintenance costs are a bit different also. C5 ZO6's have dropped considerably in value since last fall also, heck, they're even discounting the new ZO6's now. The LS6 motors and Tremec 6 speeds are pretty bullet proof also (No IMS or RMS scares). Heck.....WTH was I thinking? Oh yeah......I ALWAYS wanted a Porsche.
BTW....ZO6's on open roads get up to "Stupid Fast" pretty quickly, at least I can twist the little Boxster up now and then without dipping into triple digit speeds...JMHO.