All 3 plus more depending on build. In order of preference.
1. Reliablility
2. Modular
3. length of main power connectors
4. silence
5. number of connections
Going to have to go with A. There are other reasons, but the white really looks great and it's what would make be buy this over something like the regular colored x1050
Silence is the main reason that this power supply would be on someone's purchase list. A 1050w power supply that is silent and is made by a top tier manufacturer known for quality designs and components would be incredibly useful!
A (color): 15% - Would love to make a black & white build
B (silence): 5% - Nice, but not essential
C (SeaSonic Awesomeness): 80% - Nothing else goes in my builds these days. When you're dropping a lot of money on a system, you don't want a crappy PS to kill it all.
C: I'd buy one for build quality and performance, first and foremost. Seasonic has had a long lived reputation for building top quality kit (I have known about them about as long as I have PCP&C and FSP), so if money were no object I'd reach for them exclusively.