I think that the only reason that u would buy the 250 would be manual transmission and from what I have read their manuals are not that great. I would spend the extra and get the 350.
Comparing an Slk a boxster, I really don't know which one I would choose. I love the look of the SLK long hood short rear deck and the big star at the front. Only other I know is the SLS roadster that does it better looks wise. The boxster even with its new itineration (best looking boxster yet ) I really don't find all that great looking. I would love it for all the other reasons that Porsches are great cars. Except that with options it can cost more than a Carrera. Plus the best build it app on the web I have seen. Deciding which to buy would be a problem I would like to have.
Sorry Mazda's suck. Parts can be hard to find, and way, way, way to expensive. $1600 for a mass airflow sensor,$700 used for their van. I guess in one way the miatta does have equal value with the MB the cost of parts.