None of the alternative refrigerants are as efficient as R12 in an AC system designed for R12. I don't mean just fittings, I mean evaporator and expansion valve sizing, etc. I have R134a in the SDL. It's okay through 90F. Feels more like a swamp cooler than true AC above 100F; takes forever to cool and the blower never goes below max. I have R12 in the SE and the blower will turn down after a while. The S420 has a proper R134a system and 100F days aren't a problem. YMMV. Some folks say their R134a conversion is every bit as good as R12. I wouldn't chance it in a place at hot and humid as Houston.
Advantages - if your system has a simple to fix leak, you can fill with R134a for cheap or even do it yourself. If you're replacing the evaporator or compressor, the cost of R12 is small compared to the overall cost of the job unless you do it yourself.
Disadvantages - R134a is not as efficient a refiregerant as R12.
Ping Larry Bible. He can give you the full scoop. He might even have advice on how to make R134 work as well as R12.