Use the real R-12 if your system is still functional and leak free. The refrigerant costs a bit more but there are benefits especialy of you have a nice otherwise original car.
R12 works, it requires no modifications, you can top it off if you get low, most anyone can work on it too. The majority of replacements have issues - flamable, can not be topped off - you have to remove and replace it all if you have a slow leak, not everyone has the setup to work on oddball refrigerants, etc. My advice is to keep running the genuine R12 untill you need a major service.
If the system is broken, this may be a goo time to bite the bullet and change it over to 134a.
-Mark.