I have some time so I thought I'd share my thoughts on why the choices of my builds
First thing to note is they include a small SSD for OS/boot drive
Second, they both have better graphics cards
Third they both have better quality PSU's
Both have all-in-one liquid cooling for better overclocking
The biggest difference is in the CPU with the Alienware sporting the Socket 2011 i7-3820 while my builds use the i5 3570k. Here's why, gaming currently does not take advantage of the technology the i7 has over the i5 which is hyperthreading (the ability in essense of creating virtual CPU cores). You would be paying for something you don't need. Also my builds have the ability to overclock, not that you have to do that but it comes in handy on many games. It's good to have the option later... I can't count the threads I've read where people were disappointed to find out the system they had purchased was 'locked' and unable to be overclocked.
Again, My $0.02