Sounds like a cool project.
Anyway, for my stereo (I have a Pioneer), I definitely like having different sizes and shapes of buttons. I can basically drive my car and do stuff on the radio without looking at it. I like having a volume knob to turn, and I mostly just hit buttons for preset stations. If I am listening to a CD, I like being able to hit the next and previous track buttons. So the buttons I use most are volume knob, 1 or 2 preset stations (usually), next track, previous track. Having each of these slightly different from each other helps. Good luck with your project.