Upload & Sell: Off
I've been looking at the same problem for a few months now, and my reaction to the above suggestions goes like this:
Zeiss primes - in my too expensive compared to Rokinon saw side by side reviews and don't feel the difference is worth the extra coin
Nikon 14 - too expensive
Nikon 18mm - allegedly fuzzy on edges unless stopped down a lot, shows age, never a sharp performer, costs used more than a Rokinon 14
Rokinon/Samyang/Bowler 14 - front runner, but no filters unless you hack gel filters inside the body, not great at f2.8
Tokina 17 3.5 - interesting idea, like the size, need to find some samples and reviews on modern bodies
Nikon 14-24 - no filters, heavy, expensive but if money is no object and you don't carry gear into the backcountry like I do, yeah, the one and only UWA to have.
Nikon 17-35 - maybe
Zenitar, Sigma, Nikon MF fisheyes defished - don't like the results very much, plus my photoshop can't do it, so add a copy of DXO to the price. Do like the double use this allows, but when composing, you always see the fish world. I had a fisheye on my DX body - rarely used it.
I have a Rokinon 85mm f1.4 and am very impressed by that lens. If their 14mm is the same caliber, and from what I have seen it does deliver, I think it'll be hard to beat for being truly ultra wide and under $400. If it doesn't perform, you can sell it on ebay next week for little loss. I sold a Rokinon 8mm DX fisheye recently that I picked up on ebay, and actually turned a profit. I was amazed at how active that auction was and how many views it got.
Meanwhile I keep shooting my trusty 20mm f3.5 Nikkor - fantastic lens for the $100 it was on ebay. Small, takes filters, sharp, solid. My need for a truly ultra wide has been curbed since I got that lens, so all the high priced glass has lost a lot of attraction. I may rent a 14-24mm one of these days just to see what I may be missing out on, but I doubt it will wow me enough to swallow the price tag. I used to shoot professionally in the 90s with all the high end glass back then and know what you get when you have $40,000 of lenses in the trunk, but over the years I realized that the gear doesn't always have to be top of the line to get great results. You don't get a composition wizard included with the fancy lenses
take a look at these links, too
since I rarely shoot f2.8, the above pretty much sold me on the Rokinon, plus I really like the compact size