For practical purposes, none, to my knowledge. My impression was ASME puts an "S" before an ASTM specification to say it's then an "ASME BPVC" material spec. Each of these SA designations is listed in ASME BPVC (Boiler and Pressure Vessel Code), Section II, Part A, Appendix...
For a few more opinions, see
http://groups.google.com/groups?selm=20020808183451.26393.00002338%40mb-bd.aol.com
http://groups.google.com/groups?selm=MPG.12ddf80de564a98c98990d%40news.javanet.com