According to 16-9's review, the Nikon 14-24 is better than the Canon 14mm II prime:
Furthermore, IMHO primes are undesirable for landscapes in comparison to zooms; there are many situations where you can't change your FOV in landscape photography... zooms are indispensable in those scenarios.
I certainly hope that the past performances of Canon's ultra-wide zooms are not indicators for the quality of this rumored 14-24 But for now, I'm sticking to my Nikon 14-24 & Novoflex adapter, which gives stellar performance on my 5D Mark III. But, of course, this setup is impractical for event or fast-paced shooting scenarios.
This very well may be the case depending. I've seen some test reviewers out there posting comparison shots that would go towards proving that if they didn't botch the test/camera settings somehow. But for whatever it's worth The Digital Picture's ISO tool
tells a different story. It shows the 14-24 being much much stronger overall on it's 24mm end in general, and shows significantly worse corner distortion on it's 14/16mm end than either the Canon 14 prime or the Canon 16-35, -and then it has equal if not worse CA depending on where you look. Has arguably sharper center frame than the 14mm prime but both are suffering from some CA which is to my eye, what is causing a lack a sharpness. There is definitely a strong case to be made in both directions. I've heard Nikon's 16-35 f/4 VR is even better still for IQ, but again, from the tool comparison above, my eyes slightly prefer the 16-35LII. I would go out on a limb and say, that 'if' the above tests are reliable in any way, I would slightly prefer the 16-35 even in terms of IQ on the 16mm end, and the 14-24 Nikon easily winning these tests on the 24mm end. The test show the 16-35 performance beginning to improve again on the 35mm end.
I don't have any idea how useful this tool is for determining the finest qualities of a lens, but it is showing their abilities in some detail and overall, I would have to say, that absolutely none of this minutia matters whatsoever for my purposes. Needless to say, I'm happy Canon is bringing a 14-24 but I probably won't be in the market for one unless it does something amazing, at the very least it will be fun to rent and play with from time to time in the far future.