My biggest problem with the Zenbook Prime is Asus does not even offer an 8GB configuration.
I don't think I have seen any 13" ultrabook with 8 GB RAM beside MBA at all. It also appears most Windows ultrabooks seem to use a slighty less speced CPU with a tad lower frequency, a tad lower GPU bandwidth an no HW support for virtualization. But on the other hand, few of the ultraboos like the Samsung have stunning displays.
All in all I was also tempted by a Windows ultrabook back in July, but looking under the hood I concluded I got more bucket for the money buying an MBA.