I had a friend who was all ga-ga about the 5.45x39 round. He was determined to get an AK that shoots it because ammo was so cheap. It's a decent round but I'll still stick with the general run of the mill 5.56 because I suspect that the surplus market will dry up eventually and prices will be comparable.
As for shooting Wolf, Silver Bear, or Golden Tiger out of the AR platform, that is an absolute no-no imho. I've heard too many stories about lacquer and sealants melting at high chamber temperatures and causing problems with extraction as the 5.56 has very little taper
Now as for the 7.62x39 round out of an AR, there are some rounds that are not lacquer coated that extract very well. The only issue there is replacing the bolt carrier every 2,000 rounds or so since it has to be machined out to accept the larger cartridge base. I still have a lot of 7.62x39 ammo around because you can never have enough SKSs in the gun safe.
If you want to look at an interesting round, check out the 300 Blackout. It was designed by AAC in conjunction with Remington. At first it sounds like a wildcat cartridge but was actually developed with certain aspects in mind. It is a 5.56 case necked out to accept a .30 caliber bullet. It loads into standard AR-15 5.56 magazines and uses the normal 5.56 bolt and bolt carrier. The only difference is the size of the bore and the chamber. It carries as much energy downrange as the 7.62x39 and most 6.8SPC aficionados hate it. box prices are very reasonable and brass prices are really good. With it's subsonic characteristics I'll likely build one or two up in the next year.
Anyway, just had to drop in my $.02