Two completely different cars, apples to oranges. That being said, Porsches should be more reliable from customer experiences and friends. Mercedes are hit and miss especially the early 2000's. both are going to cost you a good penny for maint. but third party is the way to go.
not exactly apples to oranges... it's a coupe vs. coupe...
now, the 996 is a drivers car.. I've driven and raced a bunch and love them, but would I want to drive one every day? probably not. road feel and driving experience is second to none though.
the sl is luxury.... I've never had an SL, but as far as S class, i'd stay away from 99-02.. '03 is the oldest i'd go, 04 should be ok.
it all depends on whether it's gonna be a daily driver or even a regular driven car (like 2-3 days a week)... if it's a 2nd car, assuming you're getting them both at the same price... i'd take the Porsche. maintence, while expensive per/hour at most places, isn't alllll that common, but parts can be expensive..... maintence on the benz will be more frequent, and it's gonna be little stupid things like the AirMatic suspension or the trunk closure or whatever... or the gauge cluster.
daily driver - benz
weekend car/take the wife out to dinner car - Porsche
personally, i'd stick with the RWD Porsche's though, couple less parts to worry about replacing.
