http://anandtech.com/bench/product/1200?vs=699
The Athlon X4 860K is just the A10-7850K with the integrated GPU disabled, as seen in these comparisons:
=2362&cmp[]=2133&cmp[]=1781]http://cpubenchmark.net/compare.php?cmp[]=2362&cmp[]=2133&cmp[]=1781
The Athlon is going to be faster per core, but the FX is going to be better overall because of the larger # of cores -- & ignore the whole "x physical cores/2 logical cores per physical core" notation, AMD's "modules" <> Intel's Hyperthreading (although the cores technically share a floating point unit, the integer units are competely separate units, so the Athlon is a quad-core & the FX-6300 is a hexa-core). Plus, if you plan on OC'ing the CPU, I believe the FX chips are easier to OC, & have more of a margin to work with.
Biggest thing, though, is for the platform. Yes, AMD hasn't really released a "new" AM3+ chip in a while -- the FX-9xxx series just being pre-OC'd versions of the FX-83xx chips (& thus ridiculously high TDPs) -- but they're no more dead than Intel's LGA 1150 platform (which is
not going to support the upcoming Skylake processors, as they're getting a brand-new socket, Socket 1151). But while FM2+ is still being supported, the X4 860K is pretty much the strongest chip you can get for that socket, whereas the FX-6300 is one of the lower-tiers (Tom's considers it an excellent budget buy). Essentially, with the right AM3+ board, getting an FX-6300 right now is the equivalent of getting an LGA 1150/core i3 combo right now: you can save some money now, & if you need to in the future you can always upgrade to a more powerful CPU (FX-8320/8350/8370), just as an i3 can be swapped for an i5 or i7. And, since the choice of the R9 270 indicates you're probably planning on gaming at 1080p or lower resolutions, you shouldn't have any problem with the CPU.
EDIT: make sure you copy-and-paste the 2nd link -- for some reason the hyperlink didn't take the whole thing.