Don't exactly know the difference in footage. I will say that I was on the fence about whether IMAX was a big deal for Spinal Tap 2, but afterwards we all felt like it was worth seeing in IMAX. Part of why I saw it in IMAX is that it was an early showing (IIRC the day before official release) and the IMAX theater 30 minutes away was the only option for seeing it that night.

The primary reason it shined in IMAX was the concert footage; It's giving "I'm on stage at a Tap concert".