Actually it was this one.
http://www.logitech.com/index.cfm/products/details/US/EN,CRID=2150,CONTENTID=5002
I was debating on buying the first one you linked, but when with the thumb version.
You can't go wrong with the Logitech model there if you favor a thumb ball rather then the index finger type. I actually gave away the MS version of that called the Trackball Optical. You can see what that looks like at http://www.microsoft.com/japan/hardware/mouse/track_optical.asp
When I lliterally wore out the first T.Explorer that was the only model seen in a local office supply store and Circuit City alike. The T.Explorer is only found on amazon.com these days. That's where I ordered two of them. But the two things to comtemplate when deciding on which make and model is comfort and durability. Without those... you know the rest. You don't want something that falls apart real fast.