Chiming in here re: Chicago winter commuting again...
I actually tried the MEC mitts on in-store last year for the same purpose, but didn't buy them. First impression was that they were okay, but nothing special. Felt like I had to size down to get any sort of dexterity with fingered gloves under, otherwise the inner gloves would kind of just slide around in there. Didn't like this 'cos I use bar-end shifters. Another thing was that the grip material felt like it could start wearing off after a season of continuous use on bars, based on previous experience with other gloves... but this is a complete guess. The cuff adjustments were easy enough to use but I've had gloves like that where eventually the plastic tab at the end of the cord eventually pulls off from constant use. I also missed a mid-wrist adjustment on these. Again, these were just first impressions, but I wasn't crazy impressed.
Haven't tried the ORs, but I like that style cuff adjustment a lot more, and these have a mid-wrist adjustment which I prefer on the bike.