In regards to the MP 900 or Jornada 720.....it really depends on your preferences. The 720 runs HPC 2000, while the 900 can be upgraded to ce.net 4.1. The Jornada is smaller, so it's a little easier to carry around, but it also has a smaller keyboard. It's still touch-typeable, but not as comfortable or easy to use as the keyboard on the 900. The 900 screen is also a little bigger.
There are other threads here that hash over all the differences.....just do a search and you should find them.