SLK - For: Looks, running costs (of the smaller engined ones), that roof. Against: that roof means minimal storage space roof down.
Boxster - For: That engine, that handling, that badge. Against: Image - boxsters are for hairdressers, Porsches are for bankers.
If you're going to do track stuff get the boxster. Otherwise I'd go SLK, and I don't see the point of getting the smaller V6 over the bigger one, no matter what the numbers say I'd guess the running costs in real terms are no different.