There are some Camaros that have been found with a buildsheet. So you're saying that a Z28 with a buildsheet should be worth more than a Z28 with P-O-P or window sticker? I think that most people look at documentation from the point of view that it can all be faked, and if you're not comfortable with what you're looking at, walk away. Look at the P-O-P Charley posted from that L89 Camaro at BJ. But to answer your first question, if you offered me an LS6 with a buildsheet and a COPO with a P-O-P or window sticker and both had matching #'s engines/transmissions, I would take the COPO because I'm a Camaro nut.
The difference in the type of documentation wouldn't be a factor.