Springfields...
Ditto on the above reasons. If you are only going to shoot the gun occasionally -and just a few magazines of ammo, maybe it won't matter, but if you try out different 1911 guns, the differences become very noticeable immediately. The last Springfield GI model that I handled had some very sharp edges on the gun -seemed like something that would really chew up your hands and a holster too if not corrected. It seems that the GI model would be something good to buy if you wanted a base model gun to do some customizing on - to work on yourself or send to a custom gunsmith, not to leave it the way it is. Not kidding.
I use an old Springfield 5" gun that I bought second-hand and put back in working condition. It's not cosmetically gorgeous, but it's my favorite pistol. Still has the cheap factory barrel, but will hit a 2" round metal gong every time at 10 yards, and will empy a full magazine into the size of the end of a coffee cup as fast as I can shoot it.
I've fired about 6,000 rounds through it after overhauling it. Runs like a sewing machine, and would be my choice if I had to defend my life with it.
Go with the Mil-Spec. Wait a couple more weeks and save the extra money to get it. The sights make a big difference for shot consistency, and yes, the size of the ejection port DOES affect where the brass goes. The beavertail you asked about - well, it really helps with shooter comfort and recoil control. ALOT. If you had to pay individually for the extra features that the Mil-Spec has over the GI, you would see why it's a better deal. Aftermarket parts are EXPENSIVE if you buy them later and have them fitted. I know. I used to be in business doing that.