Being somewhat of a power-junky I'll vote SL500. The early (pre 92) cars are quite potent. It is high-tech, but so is about anything else desireable these days. I have not heard anything bad about the model as a whole. My driving experience is that they are sort of a poser GT. (but a world class one at that) They wont ride like your old W140, but they will be much more fun to drive. If your worried about maintenance or repair costs find a low mileage example and buy a Warrantech warranty for it. Terms and whatnot are individual specific, but for about $1500-$2000 you can get bumper to bumper coverage past 100K miles. Being that the cost of the warranty is less than the first major repiar bill (convertible top?) it can be money well spent. The V8 will not have to work as hard to move the car around. The 6 has to work much harder to move the somewhat porky W129.
Test drive, test drive, test drive. Spend as much time as you can in each of your prospective cars. After doing that, your next course of action will be much clearer. If you have any prepurchase questions we'd be glad to help.
92' Fire Breathing 500E