Steve,
If you found a great deal on the X-Terrains, get them. You'll end up on 35s anyhow, might as well get them now. As far as wheeling them on 6", try it out. You said you are cutting the fenders, just make sure they clear while your wheels are turned. There seems to be a little leaning towards limiting compression and maximizing droop nowdays and that is kinda where I would go with mine too. I have 6.5" lift with 33s and I did run 35s for a while on the street but never got the chance to wheel them. I like the streetability and overall balance of the 33s myself, so I believe its where I'll stay, but the tendency to go bigger seems to be ruling most people these days.
Let us know what you decide,
Crunch