Depends on the rest of the rig, as usual. In direct A/B on my ZDSE/DP1-I didn't think the HD800 was thinner at all. Just better really in most aspects, both technically and *gasp* musically. The TH900 is definitely the more euphonic/fun headphone-but isn't technically on the same page as the HD800. On most any other rig I've heard the Senn on-I might vote for the Fostex, as it is easier to please.
I'd agree. I find some people prefer the fun over the accuracy, but the HD-800s with the right tube-amp rig can be awesome. I think in direct comparison the result is either people find the either pair too much one way or the other.