If you are even considering Neurosurgery I would STRONGLY advise you to seek an education that does not involve the military. No USUHS. No HPSP. I am currently a 4th year AF HPSP student doing and Active Duty Tour at Travis Air Force Base in neurosurg. We have two procedures scheduled this month. Two. Not only is the service ridiculously light, there were only 2 AF spots available for Neurosurgery this year. I can only assume that the other services are in a similar situation. I am pursuing neurology, FYI.
I would not recommend allowing yourself to get into a military commitment unless you are certain you want to practice Family or Flight Medicine. There are a lot of other posters here who have much more time in the system than my 6 weeks. Read the other threads or better yet, contact some military physicians in the fields you are considering and I think you'll soon realize that there are a lot of ways to serve w/o getting leaving yourself optionless later down the road.