I am currently using a Wacom Bamboo and it has removed all wrist pain I was previously experiencing. I did have to configure it somewhat however. Convert it to mouse mode with a reasonable level of sensitivity. I then set it to have a touch on the pad being a single click and to upper rocker to be a right click and down rocker to be double click.

I didn't really consider a trackball, mostly because the table gets the advantage that nobody can use your machine which is helpful sometimes.