i originally had two 8 ways, 6 buttons each, a trackball and a spinner. that way i could play just about anything i wanted. after a while i realized i hardly ever used the spinner or trackball, so i pulled them out. then one day on the cabinet at work, my boss got me playing centipede, and i was hooked. after a while, i decided that i had to have a trackball. i then went with ultrastik 360s so i could play 4 way games effectively, without having to have a dedicated 4way stick.
i soon started realizing that 95% of the time I was playing 4way games that have a vertical monitor orientation....so therefore i got a DK cabinet and set it up with mame with only a 4way stick, 1 button and a vertical monitor. now, i hardly ever even turn my regular mame cabinet on because just about everything i want to play is on my DK cabinet. (if only it had a trackball, i'd probably never use my other cabinet)
my point is, look at the list of games you want to play, and make your panel to fit that. you can always make interchangeable panels later. more than likely you wont be happy with your first panel anyway...most of us weren't. dont worry about what your friends want to play...they will be very impressed with it, no matter what the game selection is like.