Well, I have a different view than about everyone as I have driven many of each. I happen to do all the local Volvo dealers alignments (at least the problem ones).
The two cars are about as different as night and day which makes comparisons like apples and oranges. The Volvo is front wheel drive the MB is not. The Volvo is actually bottom of the line and the MB middle even though it is the cheap option package, The electrical problems were on early eighties 240/740 models and was very similar to the late EFI injected 104 MBs. Neither problem exists of either of these models.
The Volvo is a T-Belt motor with severe castostrophic clearance problems. The MB has no such problem. The Volvo has an awfull transmission to remove and we have had to do it way too many times for that late an auto both for clutches and rear main oil seal leaks.
I personally would rather drive the MB but I know people that like FWD. I would also rather drive a newer car so it makes the decision tough.
If I had to guess both cars would have about the same expected life span from this point.
Of course I'm prejudice (bg).
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician