From reading the reviews of the R129 SL's in Frank Barrett's Illustrated Guide to Mercedes Benz, there really isn't anything different on the 600SL that isn't offered on the 500SL.
Beginning in 1991, automatically adjusting shock absorbers (ADS) and electronic traction control (ASR) were options on the 500 SL, but came standard on the 600 SL.
All SL's have self-leveling rear suspension, dual air bags, remote locking, an electrostatic air filter to capture smog and dust particles.
0 to 60 performance times were 8.3 seconds for the 300SL, 6.3 seconds for the 500, and the 600SL in 5.9 seconds.
How do you spot a 600SL? Besides the model number on the trunk, a V-12 emblem adorns each front fender vent. Because of its longer engine, the 600SL's front bumper projects 2 inches farther forward than those of the 300 and 500SL. The V-12 has a few deluxe interior trim touches, including a leather and wood shift knob and a wood-trimmed shift gate panel. A cellular phone and a CD changer are standard equipment, as is ASR control.
Basically, the 600SL is equipped the same way as a 500SL, I can't see why it would be more in maintenance, except that you're replacing two more spark plugs, two more spark plug wires, etc.
The mpg is 15-22 for the 300SL, 16-20 for the 500SL, and 13-18 for the 600SL.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".