Refilling it with R12 would cost a little less than convering it to R134a refrigerant. If you think your car is near the end of its rope then I would just go ahead and refill it with the R12, but if you plan on getting a lot more life out of your car I would convert it to R134a because it will save you money in the future (if ever it becomes low again). I don't know what all you need to change when you convert it because I have never done a conversion before. I would imagine that you would have to change the compressor, accumulator, and orafice tube, but I'm not sure on anything else.