There's another thing to consider. Holy books seem to be very hard to find, and Misty City (final holy FB) sits empty. It's actually pretty funny to enter and just walk around aimlessly in there with no worries at all.
Dark books seem to be everywhere. So it may be just me, or just for WR's. I know that Holy can end up being very powerful, but with Dark it's easier to become powerful faster. Easy is the way to the Dark Side.
Just my $.02.