Not going to UW for grad school, but was an undergrad there.
The light rails are dependable. Buses are dependable on the weekdays, but so-so on the weekends. Depending on area, I'll choose a quieter neighbourhood as high density areas tend to be a little noisy, you get fire trucks blaring past your apartments early in the morning, or some homeless guy shouting in the middle of the night, etc, you get it. Dorms are the worst, fire trucks, random fire drills , fire-alarm-triggered-due-to-some-guy-cooking-popcorn-in-a-frying-pan are common things that will happen daily. I lived in U-District and it was alright, there was noise pollution but the convenience made up for it. If you're a heavy sleeper, ignore above.
Comparing Maple Leaf vs. Downtown, i think i will go for Maple Leaf. But my personal preference would be to share an apartment in U-District, as it is super accessible to everything on campus, from the IMA (sports facility) to classes. The Burke Gilman trail, something you will hear every now and then, also runs through campus, is a beautiful jogging and cycling trail. The restaurants by the campus also open till 9pm-12am so living close by is pretty nice.