Cheaper in National, definitely. But the thing is, they only have mainstream literature, so I go to National mostly for school supplies lang.
Siyempre, the bookworm in me prefers Powerbooks; mas madaming choices, mas ok ang service, maganda ambience (lalo na the one in Greenbelt), you can actually sit and browse/read sa store, ayun. Books are more expensive though, so if meron naman sa National, dun na ko bibili. But most of the books I like aren't available sa National, sa Powerbooks lang talaga. So dun na ko.
Sorta OT: A Different Bookstore and Fully Booked are also good places to find books that aren't available elsewhere. More expensive though, pero ok narin. Wala elsewhere eh.