I really depends on the capacity of the condenser and evaporator in your car. Some cars that were designed for R-12 will not do well with R-134a. I tried "converting" one of my old benzes to R-134a several years ago and was never happy with the AC performance. I eventually switched back to R-12 and the results were much more satisfying. You may want to check a BMW forum and see how your car will pergorm with R-134a.
The main downside is cost, most AC shops are charging about $50 per pound for R-12, so a recharge will cost about $150 is you ever develop a leak. You can buy R-12 yourself, but you are supposed to have a license. In reality, you can obtain a license buy taking an on-line, open book test for about $15. There is always R-12 for sale on ebay, and (I've heard) those sellers rarely ask for proof of a license.
IMO, use R-12 unless cost is a concern.