With respects to the OP's question, personally, I'd skip out on the super zoom and go for the Tokina now. The Tokina is wicked good on cropped bodies, and built like a little tank. It's probably the best ultra wide angle lens available for DX,...that or the Sigma 8-16mm which also gets stellar reviews. I myself could live with just an ultra wide and a 50mm on DX and be a happy camper, but that's fitting of my style. The super zooms, for me anyway, are too much of a compromise option. They're not really great at anything other than covering a huge range of perspectives. They're not the sharpest lenses, they typically suffer from heavy distortion and chromatic aberration, they're slow, and the bokeh they produce is usually pretty harsh. They're good enough for good light, casual shots, but there's certainly trade off for convenience sake when you're looking for top image quality and usefulness outside of well lit areas. The Tokina will give you the goods optically, is fast, which is great for low light shots, and will open up a new perspective for you. Not to mention working within a limited range of options will also force you to stretch yourself.
I use a 16-35 f2.8 for the vast majority of my shots. There are times I'd like it to be a little longer, but not many, and I have a 70-200 for when I need it. I don't know the nikon lineup, but if they have anything like the 16-35, rent it!
Nikon has the king of all wide angles, the 14-24mm, and is absolutely worth renting, even if just to see what all the fuss is about. Nikon also have a 16-35mm VR, which is very good, and offers a few more options, (VR in a wide) but is not in the same class optically as the 14-24mm. The 16-35mm however would be a very useful range on a cropped body that works out to about a 24-52mm equivalent, and as Nathan suggests may be worth trying.
Sidebar, back to the 14-24mm for a sec, you know a lens is good when there are seemingly just as many Canon shooters using it with adapters as Nikon shooters. Just skim the reviews at
Fred Miranda and look at all the Canon shooters using it.
Also, judging by all the photographers I follow, if you were to name the top landscape shooters working with full-frame DSLR's today, I'd be willing to bet that a very sizable number of them are using the 14-24mm, even on Canon bodies. I know
Marc Adamus was/is. He had it adapted to his 1DS III before switching to the D800.
Here's Ian Plant's take on it, another top landscape photog.