Well, I currently have R-12 in my 240D and 134a in my 300D. The R-12 does seem to work a little better, and the difference is noticeable under very hot conditions, especially while idling. However, in temperatures up to about 90-95F, the 134a works just fine. I converted the 300D to 134a about 4 years ago when the price of R-12 was very high, and I needed a new compressor anyway. I'm still using the original condenser. After 4 years and 100K+ miles, the system still works fine. I have noticed the charge gets a little low over the winter, and I usually have to add a can of 134a in the spring. I may have a small leak someplace, but the 134a is cheap and easy to find so I haven't done anything about it. If I was to do it again I would probably stick with R-12, but I certainly don't have any compelling reason to change back. I don't know what your summer temperatures are like, but 134a seems adequate for summer in Colorado. YMMV.

I don't have any experience with Duracool.