Same here... no vote.
Everyone will give an opinion, including myself. What do you want as a sports car? Basically, just about no matter what you do to a 77, the C5 will do better with no mods (short of maybe 0-60 and 1/4 mi times depending how big you build the engine and tranny).
If you only want go fast in a straight line, go the C3 and make it so.
If you want comfort, go C5
If you want a year round daily driver (especially if you have to deal with inclement weather), go C5
If you want closer company and comraderie, go C3
If you want cheaper parts and the ability to do most of the work yourself, go C3
So the vote comes out even. It's all in what you want and can afford. Both cars will cost just about the same in the end. You can't rebuild a vette for cheaper than you could buy the equivalent spec and condition of the same year. Insurance would likely be the only difference, but if your record is clean, it could be a minimal difference.
If I had to do it all over again, as of a couple of months ago I said that I'd stick with everything I've done with my 80. But now that Lingenfelter has that Supercharger for the C5 for about $6300...I'd be hard pressed to choose the 1980. It's all in whatcha want. Or just go with what c4ever said... buy both!
EDIT: Either way, you're still a vette owner and in good company!