Both are highly liked in the reviews and they're similarly priced. Go by personal taste and needs.
For instance, if you plan on having it on your lap at all, which looks more comfortable on your legs? The first one might not be comfortable, and all those removable parts probably wouldn't be too convenient on the go.
P.S. I didn't read the descriptions or reviews, i'm just going by the pictures for these comments, but you get the idea.
Look at the bottom of your laptop, are there vents in more than two places? My laptop only has two vent areas so the NotePal U2 was my choice. The X3 is a higher performing cooler (that big fan does move a lot of air) but lacks the 'tuck-in' portability of the U3/U2 models. The U models have the portability thing down IMO as shown here (at 1:28) http://www.youtube.com/watch?v=NVgLL4DEn9E which is what I mean by 'tuck-in'