I switch in between both kinds of trackballs (as you say, thumb or finger) as well as using a regular mouse at times. My reasoning is that I'm trying to avoid

I switch in between both kinds of trackballs (as you say, thumb or finger) as well as using a regular mouse at times. My reasoning is that I'm trying to avoid repetitive strain injuries, so by varying my position and the fingers doing the work, I'm basically giving everything time to rest.