I took a handful on my last tour over there and can tell you that trimlines are really popular. They fit nice and snug in most uniforms (ie you don't have to worry as much about them falling out.) Also, they just feel more substantial than slimlines do. I know that is not really the case but perception is the key to happiness, right? If you think your pens are going to units outside the wire, brushed finishes are good to cut down on glare in the sun on the exposed clips when placed in a sleeve or leg pocket. Also, consider sending a refill with each pen. They can't go to staples when the ink runs out.
Also, as far as what is allowed in uniform I am not entirely sure about the other branches but I am unaware of any limitations in the Air Force. I did spend quite a bit of time with the Marines and Army my last few trips, and I saw them carrying all manner of cell phones, key rings, etc. That all being said, most people wont WANT anything perceived as unprofessional so stay away from bright colored acrylics and overly exaggerated shapes in general. A nice piece of timber is always a good call but if you really like acrylics go with something in muted earth tones.
Lastly, I appreciate everyone here who supports this kind of effort but right now you kick the most A$$ for putting the thought into this that you have. Consider sending a small note with your name and a message with each pen. You might never here back but I promise the intention wont be lost.
-Southwest