Not tried either yet, but plan to buy the Rokinon/Samyang in the near future. For its budget price, there just is nothing else to consider.
I've heard and read good things about the Sigma zoom, but it is 3x the price of the Rokinon, and has slower aperture. However, besides zoom, you get autofocus and aperture set in camera, so that's a major plus. If you are shooting long exposure on a tripod, or otherwise have control of the lighting (or you're using strobes or other lights)...then wide aperture is less important. It would be very important though, if you prefer hand-holding, and shooting in a dark room.
12mm is certainly much wider than 14mm. Whether or not you can make use of the "rectilinear projection"...which basically looks like distortion...is another matter. There are ways to correct it manually in photoshop, but you wind up with an image that has less pixels, and takes in less of an angle of view. 14mm also shows significant rectilinear projection. Your straight lines will be straight, but objects (such as people, or anything round) get stretched toward the borders and corners...becoming "fat" or "egg-shaped". Certainly there is aesthetic appeal to leaving this uncorrected...but not always.
There's even a bit of this at 24mm...but it's far less necessary to correct.
