Definitely Trackball, the larger the ball the better, I use the Microsoft Trackball Explorer, I'll never go back to a regular mouse, wired, optical, or wireless, none of them can compare.
<b><font color=purple>Details, Details, Its all in the Details, If you need help, Don't leave out the Details.</font color=purple></b>
Index fingers definitely, doesn't matter which brand, and most that swear by the regular style mouse have never used a Trackball Mouse, you don't move the mouse all over a pad, all you move is the ball, index style puts left and right click thumb activated, its excellent in CAD and pure killer in games, especially when you have to turn quickly.
<b><font color=purple>Details, Details, Its all in the Details, If you need help, Don't leave out the Details.</font color=purple></b>
<i>Mmm Dawn AND Eve at the same time...Drroooooll
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<b>XP2100+, 1Gb RAM, ASUS A7N8X, PNY 64Mb Ti4200. 😎
The most noticable diffrence will be the looks & ergonmics (eg. I've got big hands so I tend to lean towards MS as they're a little bigger than logitech equiv' items), the other thing, slightly alarming with the mice but hardly perceptable with the trackerballs and then only at low speeds, is that the logitech software changes the pointer-to-device relationship which can give a really strange sensation that the pointer isn't moving in the same way as your hand.