It depends on you. I have a buddy that used to buy the ammo first so he'd have an excuse to buy a gun.
I think if you are for sure getting one, and have noticed how scarce the military stuff is and plan/would like to have a stockpile of it then go for it. I don't think it is going to get cheaper, and I doubt it'll get any easier to find. the two military types, xm193 and m855, are pretty damn hard to find right now.
If you end up not getting the AR then wait till prices are even higher and guys can't find it (i.e. a week before the election) and sell it. you won't lose other than any interest your money is making you at the bank.