Elise is around $44K. The Boxster S is around $55K. The SLK55 AMG (keep in mind it's an SLK, not SL) is around $64K.
Much as I like the Elise, the Toyota Celica derived engine w/ only 190 hp doesn't quite do it for me. It'd have to have 250+ hp to make it worth the money. It's so torqueless that a kid in an STi or Evo would eat your lunch if you made the slightest mistake off the line. Torque can mask bad driving and that's something the Elise doesn't have.
I'm a fan of horizontally opposed engines, having owned numerous air-cooled VWs and being the current owner of a Subaru Legacy 2.5 GT, I naturally lean towards Porsches. My biggest problem w/ the Boxster S is that too many people have regular 2.5 and 2.7 Boxsters, which somewhat dilutes the appeal of the Boxster S. The Cayman S suffers less from this problem, especially since the new base Cayman 2.7 was just introduced as a 2007 model.
My wife is a big fan of the SLK in any of its iterations. That alone should knock it off the list, if the lack of a manual transmission doesn't.