I would go with Sadlak because they are a Brookfield type mount. I would go with steel over aluminum. Steel will last longer than aluminum.
Smith enterprises is also a good mount just like the Sadlak better.
Remember that the specs on you receiver may be different than the original GI specs. The $5.00 inspection kit form Sadlak sounds like a no brainer.
invest in a cheek pad for your M1A
I used to own an M1A and just recently sold it because of the bursitis in my shoulders.
As far as ammo an M1A can be picky. Just have to shoot some up and see what it likes.
Some good info on M1A/M14 mounts.
http://www.imageseek.com/m1a/M14_Sco...FAQ_070225.pdf