IAD is one of the more junior bases. If you want IAD just go to Colgan since you will end up in IAD in short time. You could still end up there if you went to LGA for Mesaba but depending on the SLI there *could be a fence that may be restrictive although I doubt it would prevent you from moving bases.

Really doesnt make much of a difference since they really are the same company on the same equipment. If you go Colgan you could likely end up in IAD faster than the Mesaba route.