I prefer the side because I shoot a Swiss. I've been able to get the magazine changes down pretty good, but there is always room for improvement:
That being said, I find the bolt release on the XCR (near the trigger) to be ergonomically faster than any left side paddle style bolt release.
When I finish seating a new magazine on my Swiss, I then have to move my hand from the mag, to the bolt release, and then to the forward hand guard, this takes time.
When you finish seating the mag with the XCR, the bolt release can be actuated with the trigger finger at the same time as moving the left hand from magazine to fore grip, instead of needing to hit the release thus saving a bit of time. With practice, I think I could be faster with the XCR on this particular drill. Keep in mind we're talking milliseconds here.