I also had this debate when I bought my Crossfire 2 years ago.
I done a lot of research and the amount of people that have had problems with the manual box are unbelievable (Jumps out of first)
MB put a lot of money into developing the automatic box and about 90% are autos.
The MB dealer said that they wouldn't even have a manual boxed car on the forecourt.
Having said that I do sometimes wish I had bought a manual.
If you are buying used and It is out of warranty I would be very careful and check that It does not jump out of first gear.
Even the one Top gear tested jumped out of first.

Norrie.