Okay I recently went through the same decision process comparing an 04 3l SE Z4 against a 55 2.7 Boxster and these were my eventual conclusions after having extensively driven both.
Interior = Z4 (if it has sports seats), as the Porsche has cheapo switch gear and a mahoosive steering wheel.
Exterior = Z4 - it just looks better from more angles, the boxster looks odd from the sides and not great from the rear.
Roof Mechanism = Z4 - it's fully automatic, the Boxster is part manual.
Handling = Boxster, vs a Z4 with runflats it's light years ahead dynamically.
Acceleration = Draw, but the Porsche feels more composed and more fun in every day driving
Gearbox = Z4 - The Porsche is like stirring treacle.
Noise = Porsche - no contest
Comfort = Boxster - no contest
Running Cost = Z4 - no contest
Reliability = Z4 - no contest, 3* more reliable than a Boxster
COST = Z4 - no contest
Overall = I bought a Z4M Coupe, which was substantially better than both and won EVERY category even when compared to a Cayman S (though the Cayman is nice)