If you really do intend on doing a fair amount of low light shooting iwth this new lens, you might want to think carefully about an f/4. The 12-24 is really a landscaper's lens- lightweight and super sharp, but not super fast because landscapes are taken on tripods and at f/8+. If you really don't use a tripod that often, and shoot in low light a lot, f/4 may not cut it for you (or you will have to be prepared to start using the tripod a lot more often). The extra stop may be worth the extra price.
Ruahrc
Believe it or not, this is a slight concern for me. My thought process in figuring out lens upgrades has been to look at my favorite shots from the last few years shooting with an 18-200 VR / 50/1.8 combo, check the focal lengths I tended to use for them, and ask myself
1) Would I have still been able to get this shot if I had a different lens? (Seems like a basic place to start, to make sure you still get the ones you already like).
2) What would I have liked to have had in that situation to make the shot better?
3) Am I likely to have the lens with me when I need it, given what I've observed about myself and how (and when) I shoot?
One picture I keep coming back to in this is one that I took during the 'snowmaggedon' storm that shut down DC
http://rennert.smugmug.com/photos/783958033_b4ZZd-XL-1.jpg
I took it handheld with a D80 at iso400, 18mm, and wide open (f3.5) with the 18-200 VR. Basically, it would have been the same shot if I had used the kit lens that comes with the cheaper nikon DSLRs now, so even the cheap glass was good enough to produce an image that was 'sharp enough' for me to like under these conditions.
Would I have gotten this shot or better with either of the f4's? If you assume that VR is worth about two stops, and my recent upgrade to a used d90 gives maybe an extra stop of low light performance, then actually, I might come out a stop behind by upgrading to the f4 in this particular instance. I'm not overly concerned about it, but this example does stand out in my mind. On the flip side, I also see potential for an even better shot with a wider field of view, so question 2) comes into play here.
Of course this argues in favor of the tokina 11-16/2.8. But the overriding concern is that, the limited range of that lens would mean I wouldn't often have it on my camera for just walking around, whereas I can see the 12-24 being something I'd leave on my camera and carry my 50mm prime in a pocket just in case. I'd rather work around the drawbacks of a slightly less capable lens that I actually had with me than know I had a more capable lens at home. The 11-16 is a superior lens, but fails on criterion 3 above.
The ultimate goal is to get to a fairly small set of zooms and primes that cover the range I like to shoot and are pretty lightweight for backpacking. Right now I think that range would probably be covered by one of the 12-24/f4s, my 50mm 1.8, and since I don't shoot a lot at the longer end, maybe just something like the cheap 55-200 VR, or else a prime somewhere in the 85-150 range.
Noting that a lot of my portraits from vacation that I took with my 18-200 were shot between 150-180mm, I recently picked up a nikkor 180/2.8 from 1986 for about $300. It has great image quality, but honestly, I think I was using that end of the zoom more out of convenience than necessity. It's hard for me to imagine carrying it with me much of the time given its size and the tightness of the field, so even it falls down a bit on criterion 3. I'd judge it as probably a mistake to have picked it up rather than a shorter, lighter prime somewhere in the 85-130 range and just gotten closer to my subjects.
If I had this range covered in the above fashion, I think I'd be pretty much set for how I shoot.
Is it safe to assume that most of you all have boiled your kits down to a similar set of just a few lenses that cover 95% of what you like to shoot? If I didn't care about the weight, I'd think a 12-24, 50/1.4, and 80-200/2.8 would probably cover all but pros, and maybe even a lot of them.