to be honest the AR might catch people off guard if it actually kills quickly close range. nobody would expect it since it's been so mediocre in the past games. if a team had a dedicated stealthy AR guy who flanked and drained people shields instantly it might be useful.
It's up to preference. I forget where, but someone took the time to measure out the kill times for the DMR and BR, and they are both 1.8 seconds with headshots. So whichever gun you are more comfortable using.
If I remember correctly, the carbine was capable of killing in 1.3 seconds if the user has perfect accuracy.
I forgot about the light rifle's stats. I remember that unscoped it had a slower kill time than the DMR/BR but faster when scoped.