How's it faster? it uses the same stuff. That price I threw out there was with the same processor, same HDD (at least in terms of speed/size. probably not brand) Both have a 1 gig video card. Both have 1080p screen and blu-ray. Also on the Alienware, I picked the lower of the options if I didn't know what the Sony had. Like I said I configured them the same way.
Since Alienware got bought out by Dell, they use the same quality components. I just don't see paying that much more for a Dell with a pretty case.
Edit: That ASUS does have 2gigs more ram and a slightly larger screen than the sony, however you are 100 over your budget. The ram is a cheap, easy fix down the road if you get the sony, but to some people the inch you gain with the ASUS may be worth it.