In reply to Fiend:
Don't get me wrong, I love On Peak Rock and I love the BMC, but never would I actually recommend it as a serious guidebook to anyone wishing to climb routes in the Peak. It's a beautiful and mysterious thing, but sometimes - just sometimes - it's not the easiest of guides to use...
That said, I do have a particular soft spot for it. As such, points will be awarded to anyone who can name the chapters the following crags are included within (without looking of course): Stoney Middleton, Wimberry, Tissington Spires, Hen Cloud, Burbage North.
In reference to the original post: if you can find out from the LMC exactly where you'll be going/staying that might help narrow things down. I'd imagine that it'll be either/or Eastern or Western Grit, as you're likley to stick around one area or the other. Either way, I doubt you'll regret either purchase...