For those who are unaware of the backstory of the Alienware Alpha, let me bring you up to speed. Back in early 2014 Valve in a blog post basically told people that the Steam Machine release window was going to be pushed back until 2015. This essentially left hardware vendors with no software, but their systems ready to go.
So what did Dell/Alienware do? They slapped Windows 8, and a very controller friendly and functional UI deemed "AlienUI" into the hardware of what was to be the Steam Machine, included an Xbox 360 Controller, and removed the Steam logo from the chassis. They called it an Alienware Alpha.
Fast forward to today, the Alienware Steam Machine specs and pricing were announced and are mostly in line with the original Alpha. It has exactly the same hardware as the Alienware Alpha- An Intel i3-4130T, 4GB memory, 500gb hard drive, and a 2GB Nvidia GPU (Based off of the GeForce 860M). With the Steam Machine you get Steam OS and the entire thing costs $450 US, where in the Alpha you get Windows 8.1 and it costs $499
Now here is where things get interesting. If you are a good bargain hunter, you can score a deal on the Alienware Alpha. I paid $265 US for my Alpha, almost 50% off the list price and close to $200 cheaper than the Steam Machine variant. I also can play all the Windows games in my Steam library, and some MMO's that are not. The fact that the Alpha is a full Windows PC makes it way more versatile than the Steam Machine. I just don't get the fancy controller. I've since upgraded my Alpha with another 4gb stick of ram and added an SSD and still haven't hit the cost of the soon to be released Steam Machine.
Do I think the Alienware Steam Machine will be a good system? Yes. With bang for your buck considered though, I think the Alpha will be the better machine. As a bonus, you might be able to find it for cheaper. Perhaps if I get to try the Steam controller I will change my mind, but for now, I'm sticking with the Alienware Alpha.