I'm also going through medical school on an Air Force scholarship. I'm going into my second year, so I haven't done a rotation at Wilford Hall. However, I was in the Air Force for a few years before medical school and had a chance to get to know a few of the orthopaedic surgeons.
In general, most recommended trying to defer if possible, but Wilford Hall is supposed to be a very good program. I think that you would actually see a significant number of total joints there as military hospitals treat 20-year retirees as well as active duty people, and San Antonio has one of the largest, if not the largest, military retiree populations in the country. I guess you will find all of this out when you get there.
I'd be very intersted in hearing how the rotation goes for you after you finish if you don't mind.