That's a tough comparison. The cars are so different that it's almost entirely a matter of personal preference. The SD is among the most reliable cars of all time but time will take its toll. You mention that the SD needs upholstery and ACC work. That's expensive work to farm out and not particularly easy or quick work to do yourself. I'll bet the suspension and steering aren't what they once were. If these things matter to you and further investment in the SD doesn't make sense, the C might be the keeper. On the other hand, consider from a financial standpoint that it might get more value in return for selling one car over the other. I expect the C with recent documented work will be easier to turn into a reasonable amount cash. An SD that runs but needs work will fetch a pittance relative to its remaining service life.

Sixto
95 S420
87 300SDL