I sold my 300SDL a couple of years ago and bought a 1992 500SEL
A couple of thoughts:
-There are "expensive" repairs on a 300SDL, but nothing like $500 Bose amps, $2000 evaporator cores and $400 wiring harnesses! These were very expensive cars new, and cost more to maintain than an SDL
-You can fix the 300SDL yourself. You'll need a good tech for at least 25% (more likely 50%) of the repairs on your W140 depending on how handy you are. I did everything on my 300SDL except for an exhaust system, and we probably split the work on the W140 about 50/50 between my mechanic and I.
-W140 is heavier, but doesn't feel it. It's much softer over bumps, and steering is lighter. SDL had better steering feel, but I love driving the W140!
-W140 feels about a foot wider putting it into the garage!
-W140 goes through brakes and tires a lot faster. Heavier car!
-Even though some may say an S320 is underpowered, it is certainly not underpowered considering what you're driving now!
-Not sure about the S320 (shouldn't be bad) but gas mileage on my 500SEL is killer! (I'd say I spend almost double on gas to drive the same number of annual miles!)
-More creature comforts. If you thought your SDL was nice to drive, wait till you drive a W140
One piece of advice.. be absolutely SURE you are ready to own a W140 before you test drive one. One common thing I've heard from W140 owners is that they fell in love with the first one they drove. From personal experience, do NOT drive one till you're ready to buy one or you'll be hooked
Having said that, I see 300SDL's around town and drool. They're much more rare (W140's are way more popular) and look like classic cars now. I'd drive a 300SDL in a minute!
Hope this helps...