Hey I just checked out Digital Storm for the first time, and they're way better than Alienware.
Why? Well for one, they're obviously cheaper. Another great thing they seem to have is a wider range of customization. Alienware always limits your hardware selection. Also, Digital Storm offers the Cooler Master HAF (High Air Flow) case, which I think is great because Alienware only gives you Alienware cases from what I understand
Digital Storm's best PC comes with a Nvidia 480 and 6 GB tri-channel ram. This is nice, because usually when you pick up a top range Alienware there is always upgrade options even with all the money you're paying, but in this case you can't really upgrade the 480, and 6 GB tri-channel ram is very satisfactory.
However, I really do recommend you build your own PC! I can tell you honestly that could save in excess of $600 by building it yourself.