I had to answer some basic questions when I did my R134a conversions. I basically wanted a functioning AC in my cars when the temps got into that Oklahoma Summer of 90 to 108 degree heat so that I could be comfortable. While our temps in Tulsa get as high as Larry's we "normally" don't get as much humidity - sometimes - but not as much. If you want your AC to make ice or you want the interior to cool down in a matter of a couple of minutes, then my advice is with Larry - stay with the R12. R134a will not satisfy those criteria. If you don't want a little noise in your AC system when the temps exceed 100 degrees then stay with the R12 also.
In my case, I wanted to be comfortable ("ice was not requried") while driving and I wanted a system that I could maintain at a reasonable cost (R134a costs what R12 used to cost and still would except for..., never mind - don't want to start on that line). I also reasoned that since I keep my house temp in the 70 to 72 range in the summer cooling my cars to a similar range should be fine. For me it is - actually the interiors ofthe cars are cooler. R134a just happens to meet my perhaps limited criteria otherwise I would have stayed with the R12.
More of my .02 (or maybe .05) here
Dan Taylor/ Tulsa, OK MBCA
'84 300D/'90 Jaguar XJ6/XJ40