IMO, I think the FX750 represents better value for money, especially if you, like me, don't intend to toy with aftermarket cables. I think the two(FX750 and FX850) sound remarkably similar and from my short audition(~30mins, impromptu visit and we had other commitment elsewhere) I cannot discern much difference between the two. From memory, they are both definitely a step from the FX700, the mids are no longer "overwhelmed" by the bass/treble and is a lot more prominent. What is a shame though is that they also kept FX700's bass, which I still find is a touch too much. Although I don't find them overly annoying from my short listen, I cannot say if this impression will stay if I listen to them for an extended period of time.
To be fair, the FX850 is new and not fully burnt-in and I was using my gf's iPod Classic, so it is entirely possible that you find the FX850 is (significantly) better than the FX750 but from my short session I don't find the difference, if any, to be immediately obvious.