If my choice is limited to an SA Mil-Spec (not a GI) or the Colt 1918 WWI model and it was going to be my first 1911, my choice would be the SA gun because it has the following features not on the Colt.
1) Lowered and flared ejection port
2) High-profile 3-dot sights
3) Magazine well bevel
If one is going to customize the WWI, the above features will set you back at least $150 from a decent smith. With the Mil-Spec, you get them from the factory. The one negative about the SA is the ILS MSH, but it can be easily and inexpensively replaced. SA's lifetime warranty is quite nice and is another plus. Colt service, in my limited experience, is inconsistent at best.
That being said, I own a (highly customized) Mil-Spec and I would buy the Colt. Why? For the rollmarks, some of the WWI parts, and the "Made in the USA" factor. Colts really shine as the base guns for customs, and I would definitely go to town on the gun. As a shooter, the Mil-Spec will be more friendly as the sights are easier to acquire and the MSH is not smooth. Of course, replacing the Colt's MSH as in the SA nets roughly the same grip. So the MSH is a wash to me.