They are the same PSU internally.
XTR is based on the G series.
Yes, that is true. Also one of the reasons why I recommend XFX in general as their series's are based on quality Seasonic topology and build-architecture. The only reason I gave another suggestion than the Seasonic is the current price. It is cheaper with rebates at this very time, thus making it a more solid option.
Oh yeah and the XTR is fully modular, the G is semi-modular, so that makes the XTR just a little bit more neat.