According to TDP, the 14 has about 6 deg greater HFOV. In practice, it feels a lot wider than 16 than the 2mm indicate. The 14 is better optically than the 16-35: sharper and less distortion. I never liked the 16-35 much wide open. It's soft (compared the the 14L, TS-E 17), and the bokeh is nervous wide open. If you intend to shoot indoors, then every mm counts. If you're outside and can move, then the focal length difference isn't that significant and the 14's advantage is allowing you to get closer and "magnify" or stress foreground features more. I tend to use the 16-35 when travelling light, but if I'm bringing more gear, the 16-35 stays home in favor of the 14 and the 17.
I use the fisheye (defished), 14, 17 and 24mm to shoot houses depending on the size of the space. Outside, the 17 is useful for large buildings, but I prefer the 24 for landscapes (especially for stitching). First evaluate your 16-35 and determine whether or not you need to go wider, and as danski0224 suggested, try the 14 before buying it. The 14 is expensive (I got mine used at a good price) and does suffer from CA (although I've had better corner performance experience with the 14 compared to defishing, unlike privatebydesign), and you might find that 16mm is sufficent for you.