Brant;
I recently went through a similar search and ended up with the 16-35 II. I strongly considered the 17-40 as well, but settled on the 16-35 II primarily for the faster f2.8 vs f4. Partly this was just for general lower light use (I take this when travelling so personally don't want to take several primes), but I was also hoping this could double for nightscapes (haven't done many, but interested to try more).
After having this lens for a few months (including one month of nonstop travel), I have to say I'm quite pleased. Overall image quality is excellent, but being a UWA I really need to watch for the perspective distortion at the wide end. Admittedly, most of my usage is at around f8 so I could do with the 17-40 most of the time, but I like the option of f2.8 when I need it. Question is whether that alone is worth the money...
As for your specific question on nightscapes, I've tried a few night shots and there is noticeable coma wide open at the edges when viewed at 100%. Not too bad if viewing smaller and it decreases when stopped down (pretty much unnoticeable by f5.6), but that somewhat decreases it's usefulness for milkyway and nightscape shots. Here is a quick example at f2.8 (note that the focus was off a bit for this shot). View at 100% on flickr to see the coma effect on the stars, particularly in the upper right.
kauai_148 by NFLD Stephen, on Flickr
Hope this helps. But in any case, I don't think there is a wrong choice with the lenses you are debating.
Stephen