Well, recently I replaced basically the whole system in my 86 300E. I had the same dilema so, I did a lot of research and decided to keep R-12. Among the reason:
1. R-12 more efficient that r-134
2. R-134 molecules smaller than r-12, more prone to leak.
3. If flushing not done correctly for conversion, may cause some problems in the future.
4. Age and mileage of my car (even for a MB).
5. If the job is done right, you will not have a leak. So, does it matter if you have R-12 or r-134. What are you saving? About $100 on r-12 (current prices). We all know, labor is where you are going to get hit the most.
Just my .02