Wow, thanks for all the responses and tips.

My dilemma is that I'm left-handed, so I'll almost always want to use the trackball/spinner in my left hand, but I also want all my right-handed friends to be able to use the cab too. What I think I'll do is create a seperate control panel for the trackball and another one for the spinner. I'm making the control panels before I build my cab, so if I'm careful about width I should be able to get a nice swappable panel system setup.
So I think I'll go with 3 panels:
1 ) 2 x Mag-Stick Plus with 8 buttons each, plus P1/P2 start (thanks for the advice about the admin buttons, I'll keep them under the panel or on the side, or use the P2 start as a shift). I've decided on 8 buttons per player because I will want to run some of my favourite playstation 1 games on the cab.
2 ) Trackball in centre, plus 3 x buttons on both sides of it (for me and my right-handed friends), plus I might have 2 mouse-mapped buttons underneath the trackball too (since I want to play Amiga games on the cab, and ScummVM games). P1/P2 Starts on this panel (for alternate play), with admin buttons underneath/on side/shifted.
3 ) Spinner in centre, plus 3 buttons each side of it and P1/P2 starts, admin buttons as in no. 2 above.
I never play pinball, but I think I'll also stick some pinball buttons on the side of panel 1 too.
Thanks again for all the advice! If you can see any problems with my proposed new setup, be nice to hear your opinions.
Jimbo