I haven't got a SL, but bought myself a CLK350 in the summer of 2010.
Everyone on the MB forums at the time convinced me to spend slightly more and go for the facelift 350 over the earlier 320 engine. I'm guessing the differences could be the same across the range - the 350 is suposedly a lot more refined and also gives you a 7 speed auto over the 5 speed auto in the 320.
The change occured in the CLK between 04 and 55 registrations.