I would have to say that the V8-powered 420 or 500 sedans would be the better choice of the two. The V-12's are very expensive to work on and offer no considerable benefit. Either if the V8s will do the dob just fine. I have done plenty of work on those 600s, and they're nothing but a headache. I service Dick Jacobs' car (owner of the Cleveland Indians), he's upping 105K on his 600SEL, and it's always in for something every other week. Camshafts (6.7 hours labor), fuel injection, idle valve, etc...And actually, I think the BEST 140 to have would be the S320. Good motor, does the job just fine--not to mention much more reasonable to fix!
'67 250SE Cabriolet
'93 300E 2.8