Buy whichever is in better shape. At this point a W126 is a W126.

I prefer the updated W126's becuase you get better motors and brakes. The SDL is a faster more comfortable car. After having owned both I think the 603 is a better motor, but the 617 is pretty good to.

__________________
2016 Corvette Stingray 2LT
1969 280SE
2023 Ram 1500
2007 Tiara 3200