The HID housing moves the bulbs back slightly to make them more focused (they sit closer to the lenses than standard H1 bulbs). The HIDs will also be fitted with cabin controlled high adjustment.
You could look on Yahoo Auctions. They will be going cheaper there. The problem is that I'm not convinced they are just plug and play. I'm fairly sure the wiring should go through the spare 'HIDs' relay slot in the engine bay fuse box...