Building a "SteamMachine", looking for hardware advice.

Hey crew,

So I want to build a new tower to be linux specific (Debian based, likely Arch or SteamOS) to game on. I haven't built a tower in a few years and I wanted to get some other gamers takes before choosing components. Really the biggies are as follows:

Micro-ITX vs Micro-ATX (preferred brand? I've always been an ASUS loyalist)

AMD vs Intel (Beefy i5 vs i7 if in the intel camp?)

AMD versus Nvidia (This is a tough one, Nvidia has had way better driver support for Arch in the past. Future VR support is also a consideration)

Finally budget versus performance options. I've done a decent amount of research on my own and have a rough idea of where I'm heading, but I just want the opinions of fellow gamers before I jump into the pool.