I'm thinking about getting an AR-15 in a few months.
They are both fantastic weapons and you can't go wrong with either. I actually have two Ar's, one Colt and one of a generic brand and am very happy with both. They are more accurate and significantly lighter but Ak's are more rugged and more dependable in muddy, rainy, or sandy situations due to their loose tolerances. You should give each model a look see