I understand it has two more alternate takes. I always thought the sound was OK...maybe because the music was so good I didn't notice
Personally, I'd save my $15, however here's the spiel from All Music Guide:
"Even if you already own the earlier, nearly complete CD, it is well worth investing in this expanded 24-bit version, but don't be surprised if it is made obsolete by a later expanded edition with additional unreleased alternate takes, rehearsals, breakdowns, and studio discussions."