What is this "or" you speak of?
Theres so much bad crunch out there that I guess I lean slightly towards fluff, since I'm more likely to get my money's worth. If a book cant do crunch right, I'd prefer they stick to fluff. Oathbound falls into this category - great setting, but the authors just arent that good at designing mechanics.
My favorite 3.5 books are those that blend the two well. Monsternomicon, Lords of Madness, Fiendish Codex 1 and 2, etc. Books that had useful mechanics AND were great reads.