Well, I have owned and flown:
Zagi 3C
Zagi THL
Zagi 400X (400 electric and as glider)
XR Combat wing.
I can tell you this:
If you build up your Zagi (like a THL with a spar in it) it will fly better (ok, not "better" - but easier) than the XR because it will float better and weigh 2 oz less... but if you build the 3C which is more comparable weight wise than I'd say go the XR.
In general I think the XR is less suseptable to tip stalls and the ol' death spiral. Don't know why, maybe the slightly larger cord at the tip???
If you are interested go here to see some mod's I did with my XR:
http://hobiehawk.com/ZEXR.html
Some might disagree with me but i have many hours on all the above planes...
Thanks my 2 cents worth..
CJS