It seems that the 'best' reason to go with a heavy is that you get a consistent barrel profile if you want to cut it down at a later date. Otherwise, probably stick with the light. (for disclosure, I've got a heavy)
I'd imagine the difference to be minimal on paper, the bigger issue, in my mind anyway, is the shift in balance over the two. I know of only two XCR's around these parts and both are light barrels so never having handled the heavy this is all purely conjecture on my part at this point.
I ordered mine with a Hvy Barrel. Not really to warried about it. Might send it back to XCR and have it fluted but for now Im not worried about it. Alli told me when we were talking about ordering from Robarms that the hvy barrel is 7 grams heavier than the Light barrel.
I may get a heavy to cut down. Right now my 11" is a light and has enough shoulder to mount the flash hider. It was cut/threaded on a CNC lathe and tested for concentricity before being cut (they commented on how straight the Robarm barrels were, I took 2 to see which one was straighter). Once I get my can I may look for a heavy.