Good luck.. Both cars are problematic cars. If you don't mind spending time at a dealer twice a year...buy one.. If you do...stay away.
..and never buy a used German car.....OMG
BMW's are known for going thru brakes once a year.
Mercedes....fuh getta bout it. Ask any true Mercedes owner about their car and they will tell you unless you're ready to deal with a lot of maintenance, stay away.
Maybe look into a M45 or Lexus..
M45's are sweet