Purity is the only difference to my knowledge. Unless the carbon or molecules change.
like i said... different labs use different carrier agents... so yes, essentially all prop is the same, but many people have had allergic reactions to certain oils... so 'prop' as a finished product can vary greatly... with an UGL moreso...
prop is really just the ester attached to test so digging even deeper, test is test... if one reacts differently its more likely to be either 1. the injection schedule or 2. the carrier agent used... with that said... taking into consideration he has ran test before, and the fact that prop usually gives a more stable blood level than longer esters, it would be safe to say that the OP can determine if the prop in question is g2g from a couple injects...