I'm goin with ARs all day!
If you think 5.56 is underpowered, get a 6.8 SPC, 300 BLK, .458 SOCOM, or .50 Beowolf upper and swap that shit out in 10 seconds.
If you want to go bigger, get an AR10 variant. KAC EMC comes to mind for an awesome battle rifle/carbine. Or LaRue OBR or a custom AR10 from GAP or other quality gunsmith for almost the same long range precision as a bolt action.
If you think the direct impingement system is too dirty, get a piston upper. Or just run the DI one with more lube..
The AK platform is nowhere near as versatile as the AR for calibers, or mounting optics for that matter.