I really, really like my tantals that are in 5.45, but if I only had one AK pattern it would be in the "original" 7.62x39. I like the fact that its a realistic deer cartridge (and yeah, I use the AK or G3 to take deer--that makes them sporting and should be fine to import...right?)
If I had the 7.62 and wanted a lighter, flat shooting companion I'd go with a 5.56 first because the ammo is sold at walmart and every else (even the monster cabela's here doesn't stock the 5.45). If you do end up with the 5.45, it is good advice to stock up while the surplus is cheap--but wolf and others are making commercial stuff that's still cheaper than 308 or 223.
One very, very, very important note on the surplus 5.45 is its corrosive. All the 5.45 barrels that I'm aware of are chrome lined, but it'll still nasty up the internals, fcg, hammer spring, etc.