I had the CLK500. Great car, and surprisingly good mpg on a gentle run (near 30 mpg), also seems to be the most reliable engine (M113 i think).
Good: Rear seats that fit people, folding in my car too, fast, economical, cool frameless doors, looks great.
Bad: interior was not best of quality (I do not know if the SL is better), e.g. steering wheel and buttons were soft and sticky from age, one or two rattles, some of them rust some are OK. I think the later the better from a rust pov.
Both nice cars but you may as well get the v8 imho as if you do less than 10k miles a year you're talking small beer difference.