Concur. I converted my '82 300D to R-134a on the basis of predictions (in the early 1990's) that R-12 was going to become unavailable. The result was an a/c system which was ineffective above an ambient temp of about 85*F. The initial cool-down time was lengthy and the performance at idle (stop-and-go traffic ocnditions) was very poor.

I switched back to R-12 two years ago. Now it's like riding in a refrigerator. I would advise you to spend the extra money for the R-12. The money you save on R-134a won't keep you cool.