If you google the question you will find that the synthetic is just a marketing gimmick. Save yourself some money and by the cheaper one. Remember it is sold by weight.
Also the term "Freon" is actually a long time trade name of DuPont's for the CFC refrigerants that they invented back in 1930.
134a is a HFC refrigerant, irregardless for years the term Freon has been used by the public as a generic name for all refrigerants.
Just like Xerox and Kleenex have been. So much for the history lesson......