It should be mentioned here that the ANSI spec covers a bunch of stuff besides impact (dust, splash, droplet, etc) and with the new (2010) Z87.1 spec you can actually have ANSI spec safety glasses that do not have an impact protection rating. The old spec had "basic" or "High Impact" but with the new spec it's either "impact" or not. So when looking at ANSI specs you need to look for Z87.1 +. The "+" indicates that it iis impact rated, and passes what used to be the "High Impact" tests. The kicker is if it doesn't have a "+" it's just not impact rated, so don't assume "basic" protection.
The Mil Spec is only an Impact rating and doesn't cover any of the other hazards.
Also, much like AR parts, "Mil Spec" and Z87.1 are what is considered the bare minimum[/] protection Scrooge McDuck has to offer his employees. A prudent enthusiast would look to exceed that standard. I use Wiley-X, Revision, and Oakley Ballistic glasses for shooting, and the improvement in protection, comfort and clarity is hard to overestimate. Don't skimp on PPE.
Brought to you by your friendly, neighborhood DOD Safety/OSHA Compliance Officer.