Both would be equally doable with no clear advantage. (Creating the dual-boot would require a few extra steps to ensure it's done properly but would give you the option to booting to Vista on an increasing basis to get used to the new OS before making the switch.)

A few thoughts that come up:

1.) After you burn the recovery discs you can safely delete that hidden partition if you wish. (The disc would be a self-contained identical copy.)

2.) Vista is not controlled by boot.ini like its predacessors, so you'd want to edit the BCD store using something along the lines of VistaBootPRO (freeware).

3.) Installing XP after Vista makes it slightly complicated as it replaces Vista's boot manager, making Vista unbootable. You'd then have to reinstall the Vista boot manager.

4.) If you have a dual-boot system, booting to XP will result in the loss of Vista's restore points. To work around that you'd have to make Vista's partition invisible to XP. (Registry edit)

Thus, the question is what's more important to you: Easy installation or coming along slowly with Vista?

Either way we're here to help.
John