I use both types of meese: an Orbit trackball and a PocketMouse Pro.
The Orbit is the most comfortable and versatile trackball I have used, since it works for both Righties and Lefties (we are talking hands here, not political bent...). My wife has a Kensington TurboRing trackball I bought for her, and I think it is a piece of crap...the scroll wheel doesn't work properly and it is rather uncomfortable to use.
However, the Orbit is not a "portable" mouse, and I think it would be a hassle to use anywhere but on a desk. The PM Pro is quite small, has a scrollwheel that works reasonably well and a retractable cord. It takes a minimum amount of space in a bag or pocket. I use it everyday with my iBook and this dedicated trackball user loves it. It may be small, but it fits my average size male hands and the programmable buttons are a nice feature.
But when working with my iBook on my lap, etc. the trackpad is the easiest way to move the cursor, and I am not usually a fan of trackpads. If I am going to be doing a lot of scrolling or work that requires a mouse, I move to a horizontal surface and use a mouse.