All I can say is thank god for USB ports and the ability to have two mouses (that's right, not mice) plugged in at once.
I worked in an IT department supporting ~200 PCs. For those users who wanted a trackball, we would plug in a second mouse and tuck it away so that when we had to work on the PC, we could work at our normal pace.
I've spent a lot of time with a trackball and I still think a mouse is faster, especially the higher resolution gaming mouses.
EDIT: The above said, if you really want a trackball, I'd personally suggest a fingerball rather than a thumb ball. If I had to choose, I'd go with this Kensington: http://us.kensington.com/html/2200.html