In order to make the M1A semiautomatic only (that is, hard to convert to full automatic), the right side rail, where the full-automatic connector would normally go, was designed to be much wider. This had the beneficial side effect of making the receiver stiffer, and the bedding to the stock more solid. I've had target shooters, that have used both the M14 and the M1A in competition, tell me that the M1A is inherently more accurate.
That said, there's a big difference between the early M1A's, made with all GI parts except for the receiver, and the current production ones. Generally, the lower the serial number, the better. I have one with a serial number below 1,000, made in Devine, Texas, before the tradename was bought by the Reese family and production was moved to Illinois. Last time I checked, these "Devine" M1A's were selling in excess of $3,000. Mine has the National Match upgrades and is practically unfired.