As a target rifle, the AR-10 might be a better bet. Stock, they tend to be slightly more accurate. They are typically a little easier to mount optics on as well. The AR-10 will be slightly more modular than the M1A, so if buying uppers in other configurations or calibers is a consideration to you, that might be a significant advantage. Some also find the AR-10 to be more ergonomic. And the AR-10 might be a little cheaper to buy initially.
The M1A is the better battle rifle. It is more reliable, robust, and durable. While harder to field strip, it has far less small pieces to get lost or damaged in the field. Fouling is contained in the gas system, leaving the receiver and bolt much cooler and cleaner.
Of the AR-10s advantages, two are very minor, one is purely personal (ergonomics), and the other has the potential to be a large issue, though most will find it a complete non-issue. The accuracy difference between a bone stock AR-10A2 and an M1A is going to be slight. In fact, only the most capable of marksmen would even notice, and even then, it would only be appreciable in very few circumstances. Both are likely to shoot 1.5 MOA or better with just about anything you feed them and with the right ammo, both will probably hover around or slightly under 1 MOA. Mounting optics on the M1A requires a mount. Good mounts can be relatively expensive. But once you got them, mounting optics on the M1A should be easy enough and not much of an issue afterwards. The modularity issue has the potential to be a major advantage if you're interested, but options for configurations and calibers aren't as varied or common for the AR-10 as they are for the AR-15. Modularity is a non-issue for most and a clincher for some. It depends on your needs.
Cost could be an issue, I recognize that. But sometimes you get what you pay for. This is one of those times. You pay more initially for the M1A, but you don't spend the rest of your life drowning your rifle and yourself in CLP, then finding yourself covered in slimy carbon fouling. You don't have to buy tools to take your AR-10 apart like you probably should for best effect on the M1A, but you will find yourself buying a whole plethora of specialized cleaning jags, brushes, and solvents for the AR-10 that are completely unnecessary with the M1A. Or you'll end up cussing Eugene Stoner and every one of his decedents while attempting to reach places on his rifle that he obviously never considered would have to be cleaned. The price for mags on the M1A is a little less than most AR-10 mags, and they are still easier to find.
Some things can't be quantified either. I've never enjoyed the shooting characteristics of the AR. The sound of the buffer tube on every shot makes the rifle sound like a pogo stick and feel like a cartoon. The M1A feels more solid. Shooting my M1A is the most enjoyable shooting experience I've ever had, and that includes time behind M2, M240, and M249 machine guns, M16A4s and M203s, AKs, an AR-50, and a couple really fine custom bolt rifles. So yeah, it may have been more expensive, esp when you include the cost of the scope mount, and it may be slightly less accurate, though more than adequate for my needs even out to and beyond 600 yards, but my M1A is a sweet shooter and I've never regretted owning it. So I'll personally take the M1A over the AR-10 any day of the week and twice on Sunday, without hesitation.
