ASR was an option on the 560 SEL in 1991. The 560 has a limited slip differential as standard equipment. All of the other items listed previously on this post are correct. Let's not forget 2 "biggies" that were standard on the 560 that the 420 does not have. 1. Adjustable tones for the horns and 2. The rear seat footrests.
Seriously, the power on a 560 is noticeably greater than what you will find on the 420 but the trips to the fuel pump are also much more frequent.