An AR will be a great omg-we're-doomed gun to have, if that is what you want it for. But...
The real reasons to buy an AR are:
-people are driving prices up with the 'omg obama the sky is falling ahhhh!!!" demand, so get one now while it's affordable
-inflation and gas prices are driving up prices on everything, including ammo
-the market is HUUUGE! for ARs at this time, so you can do whatever you want with the platform
Obama will not and cannot pass a ban for a couple years at least. He may not be able to at all. Who knows. Stop worrying about that, and get out there and simply keep it from happening, and enjoy the hobby/sport/gun.