I have never been, but I was under the impression that Rawlings was a little nicer conditions-wise, that the water was a little cleaner, clearer, etc. Plus it seems to have some amenities that Millbrook lacks.
Me, I don't have a family or many obligations that make me have to stay at home on weekends, so I'd make the trip for a nicer experience.
But you are right. For just doing drills I'm fine with Millbrook too, after all, it is closer to DC.
Basically I would never say no to either. And yes, North Carolinians are certainly invited.
One thing I don't know and maybe a more experienced person can help: Is there any sort of etiquette for these places in terms of having to keep moving, like on a golf course? Is it okay to just spend 30 minutes parked around a training platform if you're not actually in a class, or will people get upset?