Some posts on the M14 Firing Line Public Forum from some of the top gunsmith/match armorers over there are saying the M1 trigger is a better trigger for you M1A.
If you have to replace the trigger, or just want to, then the trigger of choice is the M1 Garand trigger. It fits/works and is made slightly different where the pin goes through. The M1 trigger is better supported on the pin and will last longer with less wear to the trigger and pin. The M14 trigger has additional cuts that leave part of the pin hole (not nice and round and the same size as the trigger pin) not fully supported on the pin.
There's a possibility that many M1A's have an M1 trigger in them from Springfield Armory. I'm just letting you know that if you buy a spare don't hold out for the "M14" trigger. Chances are the "M1 Garand" hammer is not only cheaper, but it is a better choice.
YMMV as they say.
My rifle has a 2 stage trigger. It's awesome. Not sure if it's a garand trigger though. :)