I was in the same boat as you back in 1994. Talked to the Army guy, he was an idiot (coincidence I'm sure but as the Army rep he caused me to walk away), talked to the Marines - very cool, very gung ho, hoorah etc. Skipped the Navy as I don't want to be on a ship. Talked to the AF and the guy was laid back, and straight with me from the get go. I think I would have enjoyed the Marines, but I'm glad I went with the AF.
Like others have said, pick a job you want from the job book, then get it guaranteed. There is no reason to leave it to chance - you will get a crappy job otherwise, because they have high turnover rate and need to fill the slots, and you'll have to console yourself with "everybody has an important job to the overall mission" - BS you'll be miserable and will get out as soon as you can. If your ASVAB is high enough you can get any job you want. I only got an 87 but it was good enough for any job. You might have to wait for a job opening. Some jobs require additional testing to qualify for, that can be a bit tougher. Originally I went in as a computer programmer (there was a second test for that, barely passed it) but I f'd up in training so got put in a different AFSC. I'm still bitter about that but it was my own damn fault.
When you get to Basic you will be stressed out...just shake it off like water on a ducks back, in the end it is just bs to rattle your cage a bit. Only 6 weeks. You'll get yelled at, so what. It's annoying, all the extreme (impossible) details expected...that is the point, to get you in the mindset of following extremely detailed instruction. Do your best, be confident. They aren't trying to break you down mentally/physically like they do in the other boot camps. Just trying to prepare a bunch of kids fresh from mama's teat for serious jobs with serious equipment. In retrospect I'm surprised at how effectively the AF get's 18 year olds to do the job.
Are you guys really talking about fitness in the AF? Man it was a joke when I was in. Just don't get fat and you'll be fine.