If you can regas with R12 then stick with it because it is a better system, although The benz seem to take R134A better than other cars I've seen.
My 190D has a R134a conversion, I regassed it when I bought it in Feb this year (I just wanted to check that it was full) and it's been fine since. I put my thermometer down the vents on the way home today. It was 4-6degC after extended idling (5mins) and 0-2degC while driving.......The dark tinted windows help alot
I'm waiting to see what happens to the new clutchless A/C systems in a few years. The compressor always runs at 5% to circulate oil in the systems, but when a leak develops the oil can't move through the system and the compressor self destructs
from lack of lubing because there isn't a pressure switch or clutch to stop the compressor when there is low gas. The end result is either compressor, filter and tx valve replacement with a system flush or an entire system replacement.
.........I think I like the idea of the $2 oring leaking and the systems shutting down, don't you?