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I have both the lenses you're looking at--the 20 and the 24. On my film cameras, I loved them both, and never noticed any difference in optical quality between them. I never did formal testing between the two, but used them both a lot, and I'm fairly picky about what I see in the images.
Really, the only difference seemed to be in angle of view. There, I tended to grab the 20 mm more--I really love the perspective it gives. On the other hand, for shooting people doing things, the 24 was my lens of choice--it was perfect for showing people, their tools, activities, and environment. I thought of it as my "story-telling lens."
Now, on my D200, the angles of view change of course, and I have little use for either focal length. They act like a 30 and 36mm lens, and I don't find the perspectives of those focal lengths very useful for my way of seeing the world--I like the very short end, and the long end, and use the middle focal lengths very little. Of course, your style may vary. But optically, they are both really good.
I have the Tokina 12-24, by the way, and have not been pleased with the sharpness. I will take the time, one day, for some measurebating, to see if objective tests confirm what I'm seeing in my images. Not one of the pictures I've taken with the 12-24 seems razor sharp, to me, and it is driving me nuts. Maybe I have a bad copy--or maybe this zoom just doesn't compare with the primes I'm used to. But I'm disappointed that I can't produce sharp images in the wide ranges where I do so much of my work.
So I'd say that either of the lenses you are looking at is likely be optically much the same quality as the other. Whichangle of view, if either, fits into your style of work is really the deciding factor.