I have the 16-35II and would make the tired old argument that if you need a flexible ultra-wide now, then this is the only game in town. I also use the 14 2.8II (really good, really wide, but not flexible like the zoom), 24 3.5II tilt shift (awesome, but lack of auto focus makes it too slow for many subjects), 35 1.4 (love this one! but again no flexibility of focal length). I would recommend that you purchase the 16-35II. Who knows when a 14-24 will make it to market and how long it will be before the price comes down after that.
Get the currently available 16-35 and enjoy shooting rather than wait for a lens that may not be available for a long time if at all.