I think what's more important than the scope is the proper mount. I read alot about scoping an M1A here before I purchased mine and eventually had good results. I went with the ARMS #18 and it appears to be holding solid after several hundred rounds.
The scope I currently use is a Nikon 6X42 fixed power and have been satisfied with it so far. I have not had the chance to use it in the field yet so I'll have a better opinion after that happens.