Here's the main differences I know of that would influence my decision (these points may or may not be relevant to you):
X3i - built in wireless, smaller, extra 32MB built-in-storage, one-touch voice memo recording
2210 - CF slot, built-in bluetooth
The X3i doesn't have a CF slot, only an SDIO slot. While most hardware is available in both formats, complex CF devices are cheaper and more readily available (i.e. wireless cards, GPS devices). I don't care much about bluetooth, I am a wireless kinda guy... but if I ever want bluetooth capability (i.e. to hook up a GPS without cables coming out of the bottom of the unit) I can get a bluetooth SDIO card easily enough for around $50US on e-bay.
There are many other differences, and many informative reviews on the web. Do Google searches for
2210 reviews and
X3i reviews, and make your decision from what you find in these writeups.