Depending on your CPU and its integrated memory controller, you might have one, two, three or even four "channels" that can work concurrently. The i5-4670K has two channels.

While this sounds like a huge possibility for performance imporovement, in practice the gain is on the single-percent level. It can be measured, but to actually feel the difference during use of the machine, you'll need something like a 50% speedup.

http://www.tomshardware.com/reviews/Intel-Core-i7-Nehalem,20...

So for a desktop machine, I'd say it does not matter at all. Buy what's more affordable, and maybe think about possible future upgrades.