Are the OGC-Simple feature access and the ISO (ISO/IEC 13249-3:2011 "Information technology - Database languages - SQL multimedia and application packages - Part 3: Spatial") significantly different (different enought to justify purchasing the ISO/IEC 13249-3:2011 PDF)?
If you are working with geometry types beyond the usual point/line/polygon/collection or are working with more than 2 dimensions, then yes, get the ISO spec. Otherwise, no. Also get the old OGC guidance on WKB for higher dimensions (or read the PostGIS doc) in case you're worried about maximum interoperability. (For 2d and simple geometries the new ISO stuff is a superset of the old stuff (and neither of them usefully describe how to handle EMPTY geometries in WKB, in 2d or otherwise)).