Jeez I hate sounding like such a fanboy but here goes. They switched because IBM's only option they provided Apple for the future of their laptop line was just

Jeez I hate sounding like such a fanboy but here goes. They switched because IBM's only option they provided Apple for the future of their laptop line was just straight up embedding G5s into PowerBooks. This wasn't really feasible at all, they were already running dangerously hot and had abysmal battery life. PPC chips were just nowhere near energy-efficient enough. It really had nothing to do with benchmarks or power or whatever (in fact the initial Core Duo chips were much slower than their PPC counterparts). I mean think about it, Apple makes next to no money on desktops, why would they prioritize power over energy consumption? Every once in a while they'll trump up some weird graph/metric like "24X FASTER" when talking about new chips in their devices, yet just about every keynote they talk up (in concrete numbers) how much longer batteries last. Hell, the latest release of OS X's biggest feature is enhanced energy management. This is one of the biggest things that makes comparing Macs to PCs kind of silly to me—if you want raw power, you should absolutely go with a Windows box. Apple loses on that every single time, and they are fine with that.