welcome to THR!
before we answer that question we need some back ground info. what is the primary use and what do you wish to achieve with it? what ranges do you plan to shoot, and what if anything do you want to hunt?
both are great rifle but we need some help here to help you.
personally i would say go for the ar, lots of mods, and upgrades to make it suitable for anything you might need it for with in reason of course. plus ammo is generally cheaper. they are great for hd, hunting varmits, traget shooting, can be made very accurate if you want them to be.
I do believe personally that the m1a is th greatest semi auto rifle ever created they are a little more pricy to get into to begin with, but they are great guns and they have there place. give us some more info and we can help you futher, in your search.