You have to decide whether you want something that is probably just adequate as a true offensive weapon, but really fun and cheap to shoot and nice to plink with (the AR-15), or something that is a superior SHTF weapon, that isn't so fun or cheap to shoot or plink with, and is heavy, and that you will only really bring out when the S really HTF (the M1-A)!
Another big factor that nobody has mentioned is this. How big are you and what kind of shape are you in? If you are not a real big guy, and not in the best of shape, then lugging an M1-A around probably isn't the best solution. The AR-15 is lighter, and would let you move further and faster. (I am 50 years old, and I only weigh 145 pounds, so if I had to choose, then it would be the AR-15, not because it is superior as a weapon to the M1-A, but because in my particular hands, I could use it in a more superior and effective manner than I could use an M1-A.)
On the other hand, if you are six feet or more, young, strong, in good shape, then the M1-A would be the better choice as a weapon.
Best Regards, Mannyrock