Well, I'm not actively involved in DJing, but I might as well give you some basic answers while I'm here.

Typically tracks are released on white before official release on a label. They wouldn't have seen a full release on a different label beforehand, rather they'd be put out as promos (as you mentioned) in limited numbers. Most of the time if there is enough interest and there are no problems (copyright, etc.) the track will be released properly later on, but there is no quality difference. It's possible that the tracklisting could be marginally different, but if there's a track you want and you can only find it on white, by all means go for it. In many cases white labels are also rarer and more coveted than retail releases.

As for bootlegs, they are different from promos, which are official vinyls pressed by record labels. Bootlegs are unofficial productions and are typically synergies, i.e., a combination of two or more tunes/vocals. Hope some of that info is useful. I can't guarantee 100% accuracy and I'm sure there's plenty more to be said, so anyone else is welcome to correct me.

starglider, you said it all.

Promos / white labels / test pressings almost always have the same quality as full releases because they are professionally pressed by the labels. Actually, a lot of the new tracks you hear the popular DJ's playing are promos... only they get them for free (lucky b*$t@rds!)

Bootlegs almost always sound like crap because they are not professionally pressed. Usually these are tunes that are rare and/or not available anywhere and someone decided to do an unlicensed pressing.

bodhichitta,
If the tune you want is on white label, grab it while you can. White labels tend to disappear fast, esp. if it's a popular track.

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tune is first release on white label or promo and distributed to big djs to generate publicity, so when it is released theres a bigger market for the tune...