oooooh, That's a tough one. I just don't know. I haven't had the Little Hornet long enough for the newness to wear off.
They both cover a lot of the same ground. The 599 is at the sporty end of the spectrum. The 919 is at the workhorse/touring end of the spectrum.
IMHO, the 599 needs some work, as it comes out of the box. The carbs needs rejetting. The fueling on mine was so poor, I couldn't stand it. I caught a lot of flack from the guys on the 599 site when I say that. Maybe mine was worse than others, or maybe they've never ridden a fuel injected bike. A K+N, a can, a Dynojet kit, a 41t rear sprocket, and this thing is smooth, responsive, and makes respectable power. Speedo is pretty close. The riding position feels just a little too cozey. The seat is too low for me. But, the paint on the frame is thicker. The seat fits better. OA quality seems a little better. I like the guages and forks. It's a prettier bike.
We like to put goodies on the 919, but truth be told, it really doesn't need much of anything. The fueling is real good with an aftermarket can or the smelly stock mufflers. I've always thought the 919 was a bit heavier than it needed to be, and the engine is easily capable of 10 more horses.
Used prices are so cheap right now, there's not much reason not to have both.