My .02...
Does a PPL give you an advantage?
It sounds like those who have gone through the process more recently than I say it helps to GET the slot in the first place. As far as once you get in to UPT, I think it does help. My experience with two friends in my class (I went through tweets JUST after they grounded the T-3). Two guys had 0 hours (expecting to go through flight screening like everyone else) and washed out before solo. Point being that if they were on the same level as those that had experience going in, they probably would have done fine. Another guy had his CFII and did very well overall, but ironically enough hooked his instrument check. Goes to show, no matter how much experience you have, if yo screw up one thing bad enough or several little things, you can still hook.
As far as getting the PPL on your own first, think about this. Research the details of the AF paying for you to fly at an FBO. They may pay for 40 hours in order for you to get the rating, but they may not give that to you if you already have the PPL. If you want to go ahead and fly on your own first regardless if the AF will pay you for it, then consider NOT taking your checkride yet. That way the AF will essentially be paying you to build time. Then once the 40 free hrs is up, take the checkride.