My camera body is a Canon 550D/T2i. Which one would be better for landscapes and why?
Canon EF-S 10-18 or Tokina 11-16/2.8. :D
Sorry. Neither one of these is ideal on a crop body as a landscape lens. The fisheye has too much distortion and would require defishing if you ever wanted a straight horizon anywhere other than the center of the frame. And defishing will cost you the edges of the frame, so it won't be super-super-wide (which is why you got it). Also the 14mm, being designed for full-frame, doesn't go as wide as your typical crop ultrawide zooms, and exhibits wave/mustache distortion (i.e., pincushion and barrel distortion combined), so you do need a lens profile and lens correction in post.
If I had to choose between the two, then the 14/2.8 but I'm a full-frame shooter who also likes to shoot superwide indoors (if you're going to be shooting at f/8-f/16 most of the time, having an f/2.8 lens is overkill). Throw in the fact that it doesn't autofocus, you have to set the aperture on the lens manually (it cannot be controlled from the camera body since there's no electronic communication with the lens; upshot: you can only shoot with it in M and Av modes on the camera), that you need to use stop-down metering, that EXIF information from the lens is not communicated to the body, and that it's a prime, it may not be worth it as a crop landscape shooter.
The $250 or so more that a good ultrawide zoom designed for crop costs may not look too expensive given all the pain-in-the-ass factors that come with a manual-only Samyang-Rokinon-Vivitar-Bower-Phoenix-Pro Optic-Walimex-etc. etc. lens. They're all the same, btw. Samyang is the Korean company that manufactures the lenses, but they get rebranded. A lot. The Samyang 8mm fisheye is the same lens as the Rokinon 8mm fisheye, the Vivitar 7mm fisheye, and the Pro-Optic 6.5mm fisheye. Why this is the only Samyang lens that's described with different focal lengths, however, is an unanswered mystery.
Footnote: on the third hand, that Samyang 8mm fisheye absolutely rocks if you want to learn to shoot 360x180 spherical panos and has the odd distinction of performing stereographic, rather than equisolid mapping as a fisheye.
I have the 14mm and use it with a T3i (still a crop body). I like it for what I get but I wouldn't consider it the best option. If time is not of the essence, I would recommend this: Canon EF-S 10-18mm f/4.5-5.6 IS STM Lens. It's only preorder at the moment, as this lens hasn't been released yet. It would, however, leave you with a lot more options than the Samyang, as it's not manual or prime. Additionally, it is built for ASP-C cameras so it should avoid the problems which were listed by inkista, but still at the $300 price point.