DISCLAIMER: not a tech.
BUT, having done research on the 560s (both SEC and SL) and reading through posts pretty much every day my *impression* is this:
420: if maintained, a bulletproof powertrain and as it has a conventional suspension requires a little less upkeep. Makes nice power, lasts a long time.
560: need to go through these with a fine-tooth comb, because very very good engineering, BUT expensive to repair. REALLY expensive to repair.
My two unqualified cents.