Which trackball to go with

I love my new IBM Thinkpad and like the fact I can use either a
trackpad or the pointer/erase type cursor but still I find these hard
to use most of the time. I did buy a optical mouse blue which is the
same as what I use at home. I find this makes my laptop very easy to
use (except for the keyboard which I discussed in another msg) but
stilll most of the time I'm happy with it's keyboard but the pointer
is the most important item to replace.
Most of the time I just don't have room for a mouse and the only other
input device I can think of is a trackball. I have looked some at
logitech but two items I am interested in by microsoft are the
trackball explorer and the trackball optical. I am wondering over
heavy use which is more comfortable? In a way I would think the
trackball explorer would be the most comfortable. It seems with the
optical the thumb would get sore from moving the ball. yet with the
optical it seems harder to reach the buttons and with teh explorer it
seems that moving the ball with the palm of your hand or as some
peopel have told me is best, the 4 fingers, is more comfortable.

I forgot to add this but I also have looked at the Logitech Trackman
Wheel Optical a lot as well as the Logitech Optical Marble Mouse. I
would like any opinion on those