Regardless of where they are built, I find my intel machine to be faster than my G4. Of course, this is obvious, but the G4 is far more stable. I've been having so many issues with my iMac Core Duo, my wireless woes cheif among them. I've never had any, any problems whatsoever with my G4 ibook, and it kind of makes me wish that Apple didn't switch architectures. I guess only time will tell if the intel machines will be just as reliable, as my iMac is a revision A. I should've waited. It's kind of disappointing, because my G4 ibook was my first mac, and its rock solid stability has given me the false impression that macs are invincible to problems. My intel iMac has debunked this.
As for the physical build quality, I don't see any difference.