ExcaliberPC's Buyer Comparison Guide (essentials at a glance)
Comparing the 6 new 13.3" Zenbooks, UX31A and UX32 series
The UX31A are all Asus "Zenbook Prime" which features IPS FHD display and SSD drives, and Ivy Bridge CPU.
Here are the key difference within these 4 models:
UX31A-DH71: Ivy Bridge I7-3517U 3rd gen CPU, 256GB SSD, Windows 8
UX31A-XB52: CPU is Ivy Bridge I5-3317U 3rd gen CPU, 256GB SSD with TPM, Windows 7 Professional
UX31A-DH51: CPU is Ivy Bridge I5-3317U 3rd gen CPU, 128GB SSD with no TPM, Windows 8
The UX32 series a bit more complex to compare, few observations, UX32 series only have 48W battery compared to 50W on the UX31A and use hybrid drives instead of pure SSD
Here are the key difference within these 3 models:
UX32VD-DH71: best of these 3 with IPS FHD, i7 Ivy bridge i7-3517U 3rd gen CPU, 500GB hybrid with 24G SSD, dedicated graphics GT 620M 1GB, Windows 8
UX32A-DB51: standard HD display(not IPS) at lower res than above, i5-3317U as opposed to i7 above, no dedicated graphics
UX32A-DB31: same standard HD display as DB51, 2nd gen CPU of i3-2367, 320GB hybrid hard drive, various accessories not included
ExcaliberPC's Buyer Comparison Guide Extended
Asus launched 3 new models of their venerated Zenbook on Dec 26th, 2012. The update is mostly to update the line to the new Windows 8 Operating System. Please see the chart below for a more complete comparison. Notable specification differences are highlighted in yellow highlight and red lettering.

Let's take a look at a few things, starting with the UX32A-DB31, the pricing makes it an extremely purchase for the entry level buyer but please remember it does not have the Ivy Bridge CPU or the IPS display and is missing the extra accessories. The thing of note is the UX32VD-DH71, it has the high end Ivy Bridge i7-3517U CPU and the new IPS FHD 1920*1080 resolution LCD panel in addition to having dedicated graphics. The UX31A line is easy to compare, all have the same IPS high res LCD and SSD though different sized SSD. The differences are in the CPU, TPM, and OS. Please also note that the battery capacity is higher than Asus global website's spec which rates the battery at 30W.
The main competitors to these 7 models are the Apple MacBook air.

ASUS 360* - 1 Year Global Warranty

ASUS complimentary notebook care package.
ASUS 360 offers a level of coverage that would add hundreds of dollars to the price of competing laptops:
One year global warranty
Thirty days flawless display guarantee
Two way free standard shipping
24/7 tech support
ASUS ADW - 1 Year Accidental Damage Warranty

Accidents happen. But when they happen to your ASUS notebook purchased in the US or Canada, you have extra protection. Whether your notebook is damaged by accidental spills, drops, electrical surges, or fire, ASUS will fix it or replace it for you during the first year after purchase.
Warranty covering accidental drops, spills, power surges, and fire damage
Target 3 business day turnaround time for repair / replacement; not including receiving and shipping time
One-way shipping (from ASUS to the user) at no additional cost
24 hour toll-free phone support
One incident covered per notebook