Just my own observations, but I felt the Lyft drivers were a little more mature and more seasoned drivers. Slightly more responsible. I would say that in the two months I was using the two companies multiple times a week, Lyft was 5 star every time and Uber averaged 4.8 (talking with friends on the phone, using inappropriate language, odd music choices like death metal). *
I was told by some of the drivers who work for both companies that Lyft is harder to work for because their screening process is more intense. I don't know if that is true, but it seems like it would make sense based on what I observed.
* I am not judging, but I get enough of this on my own, and don't need it on my way home after dealing with pre-pubescent teenagers all day