When the rear suspension gets tired on the 560SEL it's usually the nitrogen filled spheres - easy to replace and about $200/pair. The hydraulic struts rarely fail. The 420SEL used conventional shocks - no cheaper. Both ride great.
1991 560SEL cars (last year) added ASR (no spinning wheels on ice, etc) which is nice. Not sure about the 420SEL.
The 560 will have more torque than the 420, which fits well with your towing situation. Just hope gas remains cheap.
Both engines need new chain guides and tensioner, maybe cam chain also, every 120k miles or so. Other than that, given regular oil changes and a functioning radiator, they are very long lived.
Wood panelling design is identical between the cars - 560SEL uses burl wood veneer which some say looks nicer. 420SEL is walnut veneer or zebrano - I forget. All look great.
Heated seats front and rear standard on the 560SEL.