The AR 308 series of rifles can be configured in virtually any imaginable manner. Iron sights from the very basic to extremely sophisticated aperture sights are readily available. They can be fitted with red dot sights, reflex sights, ACOG type sights, scopes of virtually any type or manner. The range of sights readily available and easily installed is enormous.
The AR 308 rifles right out of the box are far more accurate than the M1a and that accuracy can be maintained easily. They are not ammo sensitive, in that they can fire any 308 ammo. They are far more easily cleaned and maintained. Barrel / bolt / trigger changes / updates / upgrades are easily done by a moderately handy owner. Accessories are available for virtually any imaginable use & are relatively cheap.
The AR 308 rifles are not elegant nor pretty or even moderately good looking, they are just plain fugly. (IMHO)
The M1a is a rugged dependable rifle. It can be gunsmithed to be quite accurate but demands special care to maintain that accuracy. It's iron sights while basic, are very good and can be modified to a fairly high level. Barrel / bolt / trigger updates or replacements demand a good gunsmith and are expensive. They are ammo sensitive for bullet weight & powder charge.
Accessories are not as widely available as the AR 308 & demand skilled installation.
The M1a rifles are good looking, rifles & can be fitted with exquisite wood stocks or metal & plastic stocks to suit the whims of the owner both at great expense.
One can build / buy & fit out an AR 308 rifle for little more than half the cost of an equivalent M1a. Parts are cheaper and far more common.
The M1a as a copy of a US military rifle will always have that "something special" as an investment or as a prized possession. Not so with the AR 308.
In the end, it depend upon what YOU want in a rifle, how you will be using the rifle and what you expect from it.
My advice is to buy both, like I did.
Trigger control + Breath control + Sight alignment = Gun Control.
NRA Smallbore Prone Master, High Power Master