I rode one with the NuVinci again a week or two ago. They are cool, but I think they might be overkill for Citi Bike. They take some getting used to, and I found myself futzing with it constantly. Since it's completely smooth, with no detents at all, it's hard to know how much you've changed the gear, or where you are in the gear range. It was fun to play with, but didn't add much to the 15 minute ride practically speaking. I think the simple three speed serves Citi Bike better, especially if the NuVinci is more expensive, much as I'd have fun playing with the NuVinci.
Is it possible the NuVinci is more reliable than the current hub gear? Do they still use Sturmey Archer? When they first introduced the "new" bikes, the gears were constantly going out of whack. They seem to have solved that problem. Did they change to a different hub?