I'm still trying to learn about the G3 universe, as I fully intend to buy one one day. They are very interesting rifles and I have admired them for a while now. Eventually I will add one to the collection, but it's in line behind an AK and another 1911.
From what I have picked up, the Century produced CETME's and complete hit and miss just like everything else Century puts out. The prices for them are also climbing, with most of them over the $500 mark now. If you get a good one that hasn't been screwed up they are great guns, but you are always taking a chance.
The original HK91's are naturally the best of the bunch, but they are also expensive and they aren't too common either. I know Springfield Armory used to have an imported G3 clone, but I don't know very much about them and they seem pretty rare today.
The option that I probably will go after is the PTR-91 G.I. model. I believe the PTR's are manufactured from original HK licensed tooling and drawings that were bought from a factory in Portugal. The G.I. model is the closest they have to original HK G3 specs, with green plastic furniture, fluted chamber, and original HK profile barrel. They seem to run between $800 and $900, with specials occasionally being put on by places like Atlantic Firearms or CDNN.