I'm in favor of running the best wire that is practical simply because the labor cost (or hassle if you're running it for your own use) normally far outweighs the materials cost difference. If you do end up needing to re-run cable in the future because the stuff you initially ran becomes inadequate, you will really regret cheaping-out on the wire itself.
But I also thought I'd point out that the reason the AC system was more efficient in a power distribution system back in "the day" was simply that it allowed the use of simple transformers to step the voltage up to very high voltages so that you could transmit high power with less loss. This is because the current (for the same power) would be proportionally lower. That low current allows the use of smaller wire, and/or reduces power losses in the long runs.
You step the voltage up at the generating site, then step it down to lower voltages as you get nearer to the end use point. And this was made possible by transformers, and transformers require AC, of course.
However, when it comes to systems like our POE devices, DC power is fine because we use switching power converters at both ends, and they work with DC power at their inputs and outputs because they create the AC necessary to operate their tiny transformers (or inductors in many circuit topologies) by switching the DC using transistor circuits. A side note to this is that the physical size and weight of these transformers can be greatly reduced as well, because the switching frequencies used can be extremely high. Transformers operating at low frequencies must be proportionally larger.
Technically, DC travels with lower losses through the wiring if all of the other variables are held constant. But the technology to perform the switching necessary to do the voltage conversions at both ends of a power transmission line did not exist back in the days of Edison and Tesla. So the AC system was obviously the superior method, and still serves us well in many situations.
So while I agree that I prefer the heavier gauge of CAT 6 cabling, and prefer to have its extra data speed capabilities, the DC being sent through it for POE purposes is actually a better idea (than AC) for today's systems now that switching voltage converters are so cheap, small, light, and efficient.
Sorry to be pedantic about this, but it's kind of interesting from a nerdy perspective.