The R129 600 brings a couple of downsides in exchange for a hundred horsies over the 500. There's 200 pounds extra on the front end compared to the 500. That affects handling. The 500 feels surprisingly lighter on its feet in comparison.
Plus the 600's nitrogen-topped hydraulic "Active Body Control" suspension is fun to watch lift the car a couple inches, and there's a button to change the damper rate. It hasn't been a horrible repair pig in my experience, but it's unnecessary other stuff to go wrong. It's time to replace the accumulators on my 600 (115,000 miles), whereas my 500 (167,000 miles) just keeps going.
Look at the prices of bmw 840 and 850s and you can see the argument that the 600sl will be worth more in value relative to the 500