The difference between an M1A and an M14 are-
Well, okay, let's step back a bit.
The difference between an M1A and a
real M14 is that the real-deal M14 is a military rifle, having either select-fire (full auto) capability, or once having select-fire capability (since disabled). This makes it a NFA weapon, subject to applicable laws and controls, and costing in the ballpark of the down payment on a nice house. They're spendy.
Anything you are looking to buy is probably going to be a civilian copy of the M14. Springfield Armory, already having shown the marketing savvy to trademark the Springfield name (despite having no relationship whatsoever to the defunct US armory), further demonstrated marketing sense by trademarking the name "M1A" (thus trading on the easily recognizable M1 moniker). An M1A is just SA's M14 copy.
Fulton Armory and many others also make their own M14 copies, each with their own name. The main differences are cost, quality of the receiver (forged vs cast), quality of the parts used (USGI vs non-USGI), quality of the barrel and quality of assembly. M14 afficionados can and will argue at great length about which is better, and which is more cost-effective, etc.
Myself? I have a Springfield Armoury National Match, and it shoots better than I do. Others gripe about inferior non-USGI parts, but either mine is an older SA (and theoretically equipped with a higher percentage of high-quality parts) or I'm just lucky. Nothing has broken yet.
Mike