I'd go with the DPMS over the colt.
Phosphate internals is simply the coating on the bolt and carrier group (yes it's what the military uses). Other coatings are sometimes used such as chrome, but it doesn't affect the operation at all and shouldn't be a determining factor. The inside of the phosphate bolt carrier group is going to be chrome lined regardless.
The barrel on the Colt is most likely chrome lined, which is good for reduced wear with high amount of shooting, but is usually less accurate. The DPMS is a stainless barrel which normally gives better accuracy.
Don't worry so much about the bolt, I'd focus on which has a better barrel and trigger, which would be the DPMS.