Logitech Trackman Wheel, hands-down.
I've used almost every variant of trackball I can lay hands on (no, never tried the SpaceBall) and Logitech won my money each time, since their first incarnation of the Trackman. Microsoft's drivers are bulky and their hardware isn't the kind of quality Logitech puts in. Some swear by the Kensington/MacAlly balls, but I like to use my thumb over my fingers. Having that many buttons might come in handy, though. I tried a couple of Microsoft trackballs, and they hurt my fairly large hands, as they're just not ergonomically well-thought-out and the button reach is so far. I hated both of those I tried, they were either jerky or unresponsive and their sensors lost the ball when I spun it. Could have been a bad unit, but the feel overall has been lousy. Logitech has never disappointed me, at least, and of the people I know who use trackballs, they're still the favourite.
My two cents