If you plan on customizing it, the Mil-Spec is worth the price difference from the GI.
If you want a stock, govt. issue type gun that you don't plan on modifiying.. the GI is the way to go. If you plan on customizing it, the $100 dollar difference is well worth getting the lowered and scalloped ejection port and the beveled mag well.. having those two things done would probably cost a little more than $100 when you factor in the fact that with the mil-spec you are also getting a stainless barrel and a little tighter tolerances.
I have a mil-spec, no complaints, 100% with factory ammo (except wolf)