On the LCD side of things, I'm nearly 100% confident the BenQ XL2410T is the fastest you can get. Its generally very hard to find a single reviewer that does a thorough test of multiple 120Hz monitors. The few reviewers that actually test things properly show it taking around 5-8ms on average to process and change pixels. The image quality is horrendous however. The panel is grossly overdriven, but remember that this is ultimately a professional gaming monitor. Performance first, Image Quality second.
From what I've researched, the Alienware seems to be one of the slower 120Hz monitors on the market. It does indeed have quite a bit better image quality than the BenQ, but again, so do 1940's television sets.
You have to decide for yourself how much you value certain aspects of a monitor. There is, and will never be a perfect monitor for everything. There have been times that I have looked at my friends old 24" MVA panel wishing my colors looked as beautiful. Other times it is reversed, my washed out colors on my 2233RZ allowed me play L4D perfectly without a flashlight (along with a slight gamma boost), while it was absolutely not possible on his monitor.