First of all, a wide angle lens on a full frame camera such as yours is between 24-35mm. The lenses that you link to are all 14mm ulta wide angle lenses. It is also important to note that they are not fisheye type but instead are rectilinear lenses.
Another important point to note is that all three lenses you mention are essentially the same lenses repackaged under different brand names.
If you in fact only need a wide angle lens, your options open up quite a bit. If you need a ultra wide angle lens but also don't mind a fisheye option, again you have more options. If you must have a rectilinear ultra wide angle lens for $500USD or less; unfortunately you won't find anything but what you have listed or other very cheap poor options.
What I would recommend if you are happy with 18mm is extending your budget slightly and buying the Nikon 18-35mm f/3.5-4.5G ED AF-S. It can be had currently for around $750 and is an excellent option.
Another option would be a bit of a DIY job but very doable. You could buy the Tokina 11-16mm f/2.8 and simply not use filters or a lens hood and still get good images at 15mm or so, but I have not tried this personally and I'm not sure if you need filters.
The Nikon 14-24mm f/2.8 and the Nikon 16-35mm f/4 VR are great reasons to buy the Nikon system in general. If you really are serious about wide angle photography; it might be a good investment to make.