Unfortunately, research aside, I think that the best way to tell for certain would be to take your chosen flat bar bike (I'm pulling for the Sirrus sport now!) for about a 10 mile test ride...then a day or two later take your chosen drop bar bike for a 10 mile test ride.
Of course, the dilemma with that is two fold:
What bike shop will allow test rides of such length?
If you have made the wrong choice, will it hurt your back at, say, the five mile away point? (A friend in a support vehicle could help with this.)
Nothing wrong with doing research. I wish I had done the kind of research you're doing when I bought my first bike...but I was just so eager! Not to say that was bad, because I still have the first, and I love all my bikes...but in all honesty that first bike only makes it out for short casual rides. (I almost feel bad for it!)
If you're still confused about gearing technicalities, etc, feel free to PM me, and I can throw down something more lengthy and explanatory for you. Sheldon Brown's website also has a lot of good stuff on it.
By the by...Semper Fi...are you a Marine? (I know it's rude to say WERE you a Marine.) Just curious. I'm a fighter pilot by trade.