This is an old thread, but I was a little surprised that no one really compared the monitors. They are quite different and serve very different purposes.
The Auratones (I don't know the Avatone clones) are great mid-range speakers. They have an incredible resolution for what they are, and they show you right away whether the midrange is correct or not. I use them all the time for mixing and mastering. I haven't heard another monitor that does what those little cubes do (and no, please don't mention the ugly NS-10).
The Auratones are indeed studio monitors, quality-wise, just with the whole focus on the midrange.
The Fostex are much fuller, i.e. more bass, but the soundstage is not as good as on the Auratones. They are great small monitors for live concerts in small venues. I will use one to support my steel string guitar in small folk duo concerts (fiddle or uillean pipes with guitar or bouzouki).
However, the Fostex do not at all replace the Auratones in the studio situation, because their frequency range is simply too extended in the lows. You will get an impression of the reproduction on bass-lighter monitors with the Fostex, but it's not at all the same as with the Auratones, which show you accurately how your midrange is sounding.
On the other hand, I wouldn't want to go gigging with an Auratone...
The Fostex can be useful to show you if there is too much lower midrange and bass in the mix, because it will sound too dull and constrained. The Auratones can show you that, too, but in a different way, and you have to deduce it from what you hear (that certain stiff sound), because they just can't reproduce the lower frequencies.
So, the gearslutz conclusion: have both.