There’s little debate amongst hard-core collector’s because they’ll have both the official bootleg series as well as underground Dylan bootlegs.
However, not everyone is a completist and will opt for one or the other. The official bootleg series is well thought out with good documentation and consistently good sound quality. These are easily available on iTunes as downloads or through Amazon, Borders and other online sellers as CDs.
On the other hand, underground Dylan bootlegs are of interest to collectors because of the number of concerts that are available. I think it’s safe to say that most of his shows from the last 10 years or more are available in one form or another. The sound quality can vary from poor to excellent. However, if you were at that show or Dylan played certain songs, the sound quality can be secondary. The problem with these is that records and CDs are hard to find. Dowloads are available from various collectors’ sites.
There you have it, two sources of rare Bob Dylan material. The choice is yours.