I've only used the Kensington Slimblade Trackball, so it's the only one I can comment on.
Ergonomics: The shape of the unit is slim (hence the name) which provides for a comfortable resting position for the wrist. The size of the unit is just about right -- not too small or big. The 4 buttons are easy to reach. However, the top two buttons have a much higher actuation force required than the two bottom buttons. I personally don't like this, but I can see why the designers of the unit desired this effect. Overall, the ergonomics are pleasantly sufficient.
Programmability/Functionality: As mentioned above, the trackball has 4 buttons. The top 2 buttons can be programmed to pretty much do anything. I use them as Forward and Back buttons within my web browser. The trackball itself has a great scrolling feature which I really like. Vertical scrolling is done by simply twisting the trackball towards the left or right. Overall, I am very pleased with the programmability/functionality.
Aesthetics: The colors used are very appealing: dark silver body, some chrome lining near the trackball housing, and the trackball itself being a deep metallic red. The body can get glossy due to sweaty/greasy hands, so I really with the body had a matte finish. Overall, the unit looks pleasing enough for both home and office use. Just make sure to keep your hands dry and the unit clean.
I'm very happy/satisfied with my purchase. If I had to purchase a secondary trackball, I'd definitely go with the CST because of its simplicity and durability.