I currently commute 3ish miles from Stadium Armory area to Metro center and ride a cyclocross bike. For me personally it depends on what your end goals are. If its strictly commuting/errand running then a flat bar road bike/hybrid style bike will work fine and every major manufacturer will make one (Trek 7.2, Specialized Sirrus, Giant Escape), but if you think you will be using it for exercise/recreation on the weekends I may suggest taking a look at a road bike or cyclocross bike. Forgive the massive read below.
I have been cycling a while but my girlfriend is the poster child for biking progression. She started out on Capital Bike share for the occasional eastern market run or the grocery trip and realized that its much nicer having your own bike. The next step was an old 10spd road bike.The plus side is it was cheap and there are a million on CL, downside is she didn't know anything about maintenance which old bikes require as well as finding one that truely fits you may not be as easy (she also hated the feeling of the old narrow road bars that came on bikes back then, less stable). Now it not to say you cant find one that fits, you can, or that you cant find one that will require less maintenance, you can, or one that you cant change parts out to fix fit issues, once again you can. Its just in her case she want something that just worked. So she moved on to a
Specialized Sirrus Sport. This bike is considered a flat bar road bike. It comes with 28mm tires stock, usually a triple crankset for making hills easy (even Congressional Hill) and a little bit more aggressive fit and feel then your typical hybrid. Now if all you are doing is commuting and errands I'd stop here. She biked many happy miles on this bike and did a triathlon, and it was the bike that she felt confident on and made her love cycling, but it wasn't long until she started noticing some things.
She started noticing on our group rides how much more effort she felt like she was doing to stay up with us. She started to want for more hand positions on the bike, the ability to move down into the drops to get out of the wind, keep hands from getting tingly etc. So she made the jump into a relaxed geometry road bike (
Trek Lexa 105 group) and has been incredibly happy. It had the drop bars and sti shifters she was looking for, it was relaxed geometry so it was comfortable for long rides, it had eyelets for fenders and a rear rack, it was significantly lighter (not race bike light but still light), and it was in her words "more fun to ride".
TL
R on this: Really think about what you want to do with the bike and choose accordingly. A flat bar road bike is incredibly utilitarian while the right road/cyclocross can also be but is more fun on the weekends but the absolute most important lesson I learned from the above story is to
buy the bike you feel comfortable on and will actually ride. My girlfriend almost stopped ridding after she got the old 10 spd because it was very much the wrong bike. Thankfully the flat bar bike reversed that mentality.
A fantastic shop that I can't suggest enough if you are looking for a used bike is
The Old Bike Shop in Arlington. This shop is run by a guy named Larry who truly just wants to see people get out and ride. I have taken many people here with limited budgets to get a bike and he has even gone as far as discouraging my friend from buy a more expensive bike in leui of a cheaper one.
Hopefully this helped.