1) Realize that the X58 and i7-9xx memory controllers are good enough to keep your cpu fed with data so that there is minimal advantage to faster ram or better timings. Think 1-3% in real application performance or FPS(vs. synthetic benchmarks which will look impressive, but are largely irrelevant.
2) Go to the motherboard web site and pick the lowest cost 12gb kit on their ram QVL list.
Or, go to the ram web site and access their configurator which will give you a list of compatible ram kits.
If you ever should have a problem, you want supported ram to avoid any finger pointing.
3) Get a 12gb kit of 3 x 4gb, not 6 x 2gb It is easier to oc three sticks, and you still preserve expansion capability. Also, ram is sold in kits for a reason. It is entirely possible that two different kits with the same part number, from the same vendor, can be constructed of sufficiently different components that they will be incompatible, yet they individually meet the advertised specs.
4) Do not pay much extra for faster speeds, better timings, or fancy heat spreaders. Only if you are a competitive, record seeking overclocker would such be worthwhile.