R12 is so cheap in today's market, the difference in R12 vs. R134a is not much more than what the kit would cost. If anyone tries to sell you R12 for more than $20 or $25 a pound, tell them to go pound sand.

In addition to the cost issue, statistically R12 systems converted to 134 last about two years before attention of some sort is necessary. If your system was designed for R12, you are MUCH better off with R12. If for some reason you are absolutely bent on using another refrigerant, make it 134 and get ready for the reduced cooling capacity, reduced long term system reliability, and increased long term cost.

NOW, get ready for an onslaught of suggestions and endorsements for every kind of junk refrigerant you can imagine. By this time tomorrow, this thread will be full of them.

Good luck,