A tough one to answer, given that you are looking at 10+ year-old cars. The quality of previous maintenance and the driving styles of previous owners are more at issue than design idiosyncrasies. However, I would be more comfortable with a 420 SEL or 560 SEL if I knew that the timing chain and timing chain guide rails were changed recently (or if the price of the car was adjusted to allow for the cost of replacement of these parts). These (V8) models have a long timing chain that runs a relatively convoluted path. A worn chain will be loose and could either jump a few teeth on the camshaft drive gears, or slap against the plastic guide rails (which become brittle with age), break the rail and jam a piece of plastic between the chain and other components, breaking the chain. Either event will cause engine valves to open at the wrong time and crash against the pistons. Expensive. While the timing chain on the 300 SE's inline six cylinder should also be replaced as a preventative measure, said chain has a much shorter and less convoluted run and is PROBABLY more durable and cheaper to replace. Dirty oil wears these chains out prematurely.
I'm not trying to frighten you off the V8's, but to give you a heads-up that for either vehicle, a good preventative maintenance regimen, while by no means cheap, is much more economical than doing nothing and waiting for a breakdown.