Personally, I use the Competition stick/buttons combo on my cab. I definately prefer the Competition buttons over the Ultimate, but I may be switching to the Ultimate stick (or at least trying it out). The only difference, to my knowledge, is the size. Competition sticks stand taller out of the cab than the Ultimate does.
The Ultimate buttons are smaller and have that concave style to them, just doesn't feel right to me. The Competition buttons are larger and have the "dome" look of your standard console controller. Much better in my opinion.
So I recommend the following:
Ultimate joysticks.
Competition buttons.
The 1P and 2P Competition "Start" buttons for that classy look. <IMG SRC="smilies/tickled.gif" border="0">