Do all of these 4k monitors do decent 1080p upscaling? That's my only real concern, since nobody's going to be gaming @ 4k native for quite a while, so the upscaling should look good and have negligible impact.
I like mini-itx and micro-atx systems these days, and am not a fan of show-off plexiglass sides, so I'd choose, in this order:
1) Falcon Micro (microatx)
2) Velocity Micro
3) Xotic PC