I have owned and shot both. If I had to go to war (especially in the Alps!), I would choose the SIG as it is the more reliable system... The SIG has a better factory stock and the folding mechanism locks up tight. The XCR has some small issues with small clips and screws that can come loose and disable the gun (locktite will cure the ejector screw problems)... I hear most of those small gremlins are now cured on the new rifles. Overall, the SIG is better built.
Otherwise, I would choose the XCR because it is more versatile: you can change caliber, it has more mounting points (especially for optics), you can put on a collapsible stock or a folder if you can find one, it takes AR mags, it is cheaper, etc, etc. The bipod on the SIG is OK but if you seriously need a bipod, you can install a GOOD one on the lower rail of the XCR.
The XCR is WAY superior in one department: ergonomics. Everything is better on the XCR: the safety placement, the cocking handle, the mag release, the reload procedure (you have to rock the mag in a SIG), etc. The SIG is heavy and so is the XCR but if you install a thinner barrel on the XCR, the balance would greatly improve...
As far as accuracy goes, the nod would probably go to the SIG but only if you use the proper Swiss ammunition or heavy bullets as its rate of twist is designed for the heavier, longer bullets... With regular NATO spec ammo, I could not declare a clear winner between the two.
I find the XCR easier to clean.
I vote XCR