Any difference between compilations and originals?

Possibly...They could be remixed. No way to tell unless you buy and compare to the original.

I've even seen compiliations where certain songs were live versions, or later second recordings, not the original studio hit.

Compilations are usually put together on the cheap (the whole point being to resell something you've already sold), so I'd be surprised if there were too many remasters on them. I doubt the K-Tels of this world even have anybody on staff qualified to do that.

The quality of the compilation, then, depends on the "masters" they used. And I use the term advisedly, because there's no guarantee they didn't just tape if off a 45.

That said, reissue labels like Rhino often take care to find good-quality masters, so it really all depends, and you can only tell by listening.

I usually don't bother buying compilations unless either this is the only item of particular performer that interests me or the compilation might contain single items that were ever not part of any of one's album...
Anyway, as to the quality, I'd rather put everything on scale where I would weigh my desire to rather listen to the music rather than to its quality in all of format and media cases.

It is a Rhino collection, but I read on cdnow that some of the tracks are edited(DAMN!!). Im having second thoughts because for the same amount of dough I could pick up about 4 greatest hits cds by some of the artists.

I might be wrong,but compilations are skewed because they try to correct for the recording levels which the masters were recorded at making the compilation terrible.

Hard to get that right witout messing the whole thing up.