If you were talking about the Sony U50, then it would be a different matter
Don't have a UX50 but know people that do...
1. Built in WifI/Bluetooth.
2. Very small package.
3. Bigger (easy to access) software/peripheral base.
4. Camera (But you might as well get a camera phone)
1. Don't like the keyboard, flush, hard to press buttons (compared to zaurus).
2. You have a choice of Landscape with keyboard or Landscape without.
3. Charging solution is a pain (I'm told). Friend mostly carries it around with a 3rd party cradle.
4. Sony Propriatry tech. (reinvent the wheel our way Memory Stick).
1. Excellent Screen with double the screen res. 640x480 v 480x320
2. Both Landscape and Portrait
3. Much better keyboard
4. Better expansion flexibility with CF + WiFi
5. Some serious "Heavy Duty" applications available
1. No Built in Wireless
2. Since SD is memory only, can't use 2 peripherals at the same time (can be problem with no WiFi/bluetooth)
3. Software side fragmented. No "Unified ROM" to give access to standard and X environment.
For me, the Zaurus wins Hands Down. I can "live with" lack of built in WiFi/Bluwtooth.
Only thing I really miss is the lack of a good Navigation application for use with my GPS unit.
If it doesn't have to fit in your pocket and you have loads of money get a U50/U70.
Sorry, currently playing with them, so cool...