For all intents and purposes, the Ninja 250R/300 is not a sport bike. It just looks like one. It's an outstanding commuter. I may catch flak for this, but they're basically scooters, just more maneuverable, sexy (most girls won't know the difference
), and safer (more power to keep up with traffic and if you need to get out of the way). HOWEVER, even though I stated they're basically scooters, I'm buying another; they're too much fun.
They're one of the best, if not the best, beginner bikes. They're great on gas (60+MPG), parts are relatively cheap, the engines are near indestructible (assuming you keep up with the maintenance, which again is cheap), and insurance is affordable (even if you're young).
Go with a 250R. You don't want a new bike, at least not at this stage in the game. It sucks dropping a bike (you're going
to drop it; either in the driveway motionless or out on the street); dropping a new bike sucks even more.
If you go to the dealer, you'll be stuck paying their fees. Bikes also tend to cost more at dealers compared to private sales. Try to find a used 2008-2012 (stay on the lower end of that spectrum; they're cheaper) on Craigslist.
You have to present it to her as you doing it the "right" way--having your safety be number one. The Motorcycle Safety Foundation course comes before anything. That course is worth every penny and you won't find someone on here who does not recommend it. Even some people on KF who didn't take it when they started out years ago, say the wish they had.