I'm not an expert but here is what I currently understand.
Pre about 1991 Springfield used GI parts which were forged not cast. After that they went to casting their own. Getting a forged bolt, op rod, triggers and other parts are really nice, but unless you are shooting more than an averaged of 100 rounds a month probably not worth the extra $ (in my overly analytical opinion).
The recoil in an M1a is quite a bit more than an AR 308. The shorter the barrel the greater the recoil on M1a's. Also many of them have their front sights machined into the flash hider making it much more pricey to switch it out for a muzzle break if you feel so inclined.
308 really shines after 300 yards. I have really good eye sight and that is about where my vision starts to suffer without optics. M1a's can have an optic mounted but the mounts are expensive and the stocks are not designed to have raised cheek piece. Thus if you plan to shoot long distance as smaller targets, the M1a is not the best option. If you are just shooting "cans" under 300 yards then why not go with a more economical option...
Don't get me wrong I love my m1a match, it shoots amazing well. Much better than I can shoot it with open sights. However I hardly shoot it because my AR in 308 is just much more effective and ergonomic.