First: Don't limit your choices to just the Air Force. Consider other branches including the U.S. Public Health Service (Naval commission - but work mostly for the Department of Health and Human Services... still can't place them? Think the U.S. Surgeon General). http://www.usphs.gov/
They JUST entered into a huge partnership to assist the DoD with exactly what you're talking about (link for it is on the website).
No matter what branch you choose, you would be getting a direct commission with an advanced rank (Master degrees get O-2 and Doctorates get O-3), plus since you are prior enlisted, you would also have an "E" after your pay grade until you make O-4 for pay purposes. That means a bit extra base pay (look at the bottom of the Officer's pay scale - footnote 3 - to see what I mean), and even a little more for the housing allowance. http://www.militaryfactory.com/military_pay_scale.asp
As far as advancement potential - the Medical Service Corps folks are all promoted by separate boards (no matter the branch of service) than the "regular" line officers, so not being a Pilot or Academy grad won't inhibit your promotions. Keep in mind, though, that some specialties will only have one slot in flag billets (like there is only one Surgeon General per branch - and that's a three star billet; top dentist is two stars; top pharmacist is one star; etc.) and that's PER BRANCH. So while it's not impossible to be a flag officer as a Psychologist, it's not probable - BUT O-6 is certainly within reach - and your prior enlisted knowledge will only help you.
A direct commission (which the medical services for ALL branches qualify for) means no OCS/OTS, but you will face "indoctrination" training. Think a glorified charm school for two to four weeks (THAT depends on the branch). You'll be there with a bunch of nurses/docs/lawyers/etc. learning how to salute, wear the uniform, a little about the U.C.M.J., do a little P.T., etc. It's a breeze and since you're prior enlisted, you'll probably be made a class leader of some sort.
As far as choices for a home-base - same applies as when you were in before. Fill out your dream sheet and hope for the best. The Service's needs come first - and while you may be a Phychologist, that's just your career field. You're an officer first. With the Air Force/Army/Navy, you could be stationed anywhere (As a P.S. here - the Marine Corps DOES NOT have MSC folks. They are Navy folks. When they are assigned with a Marine unit, they can wear the Marine uniform, but look closely, the symbols - buttons, devices, etc. - all have either the Navy or PHS insignia, not the Marines').
With the PHS, it depends on which agency hires you inside the DHHS, and it will be stateside (you have a lot of say with the PHS, but even here smart choices will impact your promotion potential - they want to see diversity in assignments, not someone who's in it just for themselves).
Hope all this helps. I'll keep checking back to see if you post any follow-up questions.
Edited to add:
In response to the poster(s) who said the Air Force doesn't accept prior service. Yes, they do in fact accept folks with prior service, esp. medical officers (and this person would be an MSC officer as a Psychologist). Like all branches, though, you can't have more than 8 - 10 years without getting a waiver (you have to be able to serve a minimum of 10 years commissioned to retire as an officer).