I can't really tell you what to do, but I can share my experience.
First I bought an M1A. Why did I buy an M1A instead of an AR? I have no idea. I guess I just wanted an M1A and hadn't paid much attention to the AR. So I have an M1A and I shoot it fairly often and at NM highpower matches. That is the most fun you can have with your M1A in my opinion. Shooting iron sights at a target 600 yards away is a blast.
I eventually got a Garand too, because everyone should have a Garand. I find that anytime I want to have a "travel across the state" gun in the trunk, I grab the Garand. I guess it is just because the Garand is simpler and the ammo is easier to haul about. Maybe it is because the Garand is cheaper and I could always get another. Whatever reason, I have found that since I had my Garand, the M1A has sort of taken 2nd place.
Now I live in the the PRK, so I can't get a real AR, but I bought a FAB-10. Shooting the .223 vs. the .308 is way cheaper! Most of my .223 loads take half of the powder as my .308 loads and the bullets are cheaper as well.
First, if I were you, eventually buy both an AR and M1A. Why? Why not? Plus since I can't get a FAL, get one of those too as a way of honoring my struggle here in the PRK.
Since your budget is strapped, you have two options really.
1) Buy the M1A now because you deep down inside want it more. Also, I have the feeling if you buy the AR first, you might lose your desire for an M1A. Just realize the ammo is going to cost a little more. However, that depends on what kind of ammo you want to shoot. There is a lot of military surplus .308 on the market right now, but it is sometimes corrosive and always berdan primed so you won't be reloading it. If you want some match ammo, then by all means it will cost more. You also probably will have a harder time shooting in in national match highpower as well. However, if you are a reloader, most of your .308 components will match up with the .30-05 components. This is a plus.
2) Buy the AR-15 now. The ammo is cheaper all the way around, you can probably do better on the highpower circuit with a good AR. You already have a .308 caliber battle rifle in the Garand. Just be cautious that you might not see a need for the M1A later. That is why I think option 1 is best. Once you have an M1A, you probably won't want to get rid of it, but you might want to buy an AR-15 too just for the fun of it. Plus don't forget to get a good FAL because the DSA FALs look cool, the magazines are way cheap, and because I can't have one so you should.
Good luck!