Due to the magazine situation, I'd say get the 5.56 for the best reliability. Even though ASC has produced good mags in the past, their new ones have been questionable in terms of quality and reliability. However, all that relates to US mags....not sure of the mag situation in Canada.
I have a 8 5/16" bbl 5.56 mini and a 10" 7.62x39............The 7.62 definitely packs more punch, but again, if you can't get reliable mags, it really doesn't matter. Luckily I have over 20 reliable ASC mags....but I'm not sure of the current status of ASC at this point.
I use mostly Brown Bear non-corrosive ammo in 7.62x39. Wolf I've found to be less consistent, but still runs reliably (in terms of extraction) in 7.62x39. I also have quite a bit of Golden Tiger 7.62x39 that works just fine as well in the XCR but seems to be lighter loaded than Brown Bear.