Well... if I were choosing and aside from the trackpad vs. trackball, my choice would be the 8900. Just because that is what I was choose, doesn't mean it should be your choice. Things to consider:
Trackball vs. Trackpad
Network (Tour is only on Sprint/Verizon in the US)
Features (I have the Tour, had the 8900 and miss the heck out of wifi)
Price
Keyboard (try them out and see which one you'd feel more comfortable typing on)