+1 to the grats on joining the military. Good luck in basic!
I did a bunch of research on this same topic, and what I came up with is that the 500 is really dated technology. I also think it looks ugly! I’ve read a ton of forums comparing the 250r and 650r as a first bike, and read success stories for both. I personally got a 650r as my first bike, and so far so great! I would heartily recommend it HOWEVER I think it depends a lot on your mentality. The 650r was a longer term investment for me; I wanted something exciting that had more power than I needed. I wanted to learn on something I could grow into, that was also accessible to a novice. (Unlike, say, the 600RR that the jackass at the dealership tried to sell me. WTF???)
I think it comes down to an honest assessment of your goals and your personality. Can you know that the bike has power in the bank that you won’t use for some time, and slowly work your way up? Or do you want to master a smaller bike that you can push to its limits sooner, and that has a hard cap on how much trouble you can get yourself into?
The 650r has a very smooth power band (not that I have much to compare it to), and if you are in a cautious and thoughtful frame of mind, you can keep yourself safe on it. However it also has the power to respond to dangerous thinking with dangerous action.
Also, be careful about subconscious thoughts along the lines of “I’m about to be doing [xyz deadly military thing], so what do I have to fear from riding?” Remember lawrence of arabia died on his motorcycle.
Be safe, and try to be honest about what you will do with the power in your hands!