Like you I have bought a fair few 12" Singles, and generally found the quality to be a big step up.
....can't always say the same of 'HiDef' downloads.
I will probably get shot down in flames now ....but, my belief if that a 12" single is the same track on different media - ish.
With HiDef there is a whole panoply of subtleties involved:
Is the HiDef version the same tracks but with higher sampling/bit rates?
Has it been re-mixed?
Has it been re-produced?
Was the source recording suitable for handling as HiDef
......etc.
And this is before we get into when IS HiDef high definition.
Personally I definitely read reviews before buying anything from HDTracks.
I have just bought the Peter Gabriel 24bit 'So' remaster, and in all honesty don't think it is any great step up from the CD re-master.
What I DO think is fantastic is the 24bit live So concert from 1987 that is available from Society of Sound.
For me it comes down to the quality of the recording, and in this way I think that buying a 12" single was far more transparent ....but it was ONLY a single.
M