You might find a bit faster career advancement in the Reserve. My unit, the 6th JAG out of Seattle's Fort Lawton (but soon to be at Tacoma's Fort Lewis) always needs 27Ds. I imagine the other LSOs are the same. We promote 'em fairly quickly too. Having said that, I've been Reserve status only, and cannot tell you what the AC promotion rates are. Slyjackalope observations are spot on. I know of only one 27D who left, to take a Transp butterbar commission, but he's going to law school and coming back as a 27A if he can.
Crunch is right--call the JAG Center and Law School (TJAGLCS) in Charlottesville, and see what they say.
A man has two reasons for doing something: the reason he tells you and the real reason. (J.P. Morgan)