the main difference is that WM5 uses rom to store everything so a) you can't lose things if battery is empty and b) battery lasts longer.
WM5 has lots of feature changes but if you read the reviews you will see that it's an X50 with much more storage & a new OS. My view is that anyone who does not need to buy right now should wait a few weeks to get the first end user feedback on the x51 but I'm certainly intending to buy the x51 to replace by X5.
