I've had the older JF version and the SG version. Both were great, but I kept the SG. It is just massive, greasy, high gain goodness. The blue/red channel modes and the structure switch interact with each other in some very interesting ways on this module. I like the red channel best with the higher-gain structure and the gain set at about 10:00 (this module has a TON of gain). When I was first playing it and I tried the same settings on the blue channel, it seemed almost uselessly trebly. I eventually realized that the treble rolls off really nicely as you increase the gain, so the secret on the blue channel is to use the lower-gain structure and compensate by turning the gain knob up higher.
The difference between the two gain structures on the SG XTC also seems to differ from the JF version. On the JF version, it behaved more like a gain boost, but on the SG version, the change in feel is more noticeable (to me, anyway), and the difference in gain level isn't quite as great. On the SG version it feels more like it's a difference between a fluid, compressed sound vs a more open sound than it does a gain boost, even though the amount of available gain does change.
One big caveat to all this is that Rob has revised his XTC design since I had mine, and supposedly this had a big impact. I really don't think you can go wrong with either, but if that SG version is still in the classifieds, I recommend snapping it up, because you won't be able to get a new one built.