It may depend to some extent on where you live and how much longer you plan to keep the car on the road (forever?). If you live in Barstow or Houston, you probably want as much cooling as you can get and maybe ought to stick with the R-12 system even though future repairs to the system are going to cost a fortune. If you live in Seattle or Caribou, Maine, there just aren’t that many days a year when you will need killer air conditioning and an R134a conversion may be more than adequate even if it turns out not to be quite as good at cooling and dehumidifying.