It's up to you. Everybody's priorities and personal tastes in framing are different. Which lens is going to be the better fit for you depends a lot on what/how you plan to shoot (handheld or on a tripod), whether you plan to move to a full-frame body in the near future, and how wide you want the lens to be able to frame a scene.

The Samyang/Rokinon lens is designed for full-frame. It has a maximum aperture of f/2.8, and so may be better suited for handholding or environmental portraiture (indoors) and low light. But it's 14mm, which on a Canon APS-C sensor, has equivalent to a 21mm lens on full-frame. And it doesn't zoom. And if you're shooting landscapes, there may be a physical barrier (canyon, cliff, wall, etc.) that may make it difficult to "zoom with your feet" if you want a wider view.

The 14/2.8 is ultrawide on a full-frame body, but it's only very wide on crop. Ultrawide with APS-C tends to start at 12mm. And the front element is convex (bulbous) and juts out so there's no way to use a screw-on filter with it; instead, it requires a special filter holder. Landscape shooters often may want to use a polarizer or graduated neutral density filter.

It is also a manual lens that doesn't have any way of electronically communicating with the camera body. You not only have to set the focus manually with a lens ring, you also have to set the aperture that way, too. Your 600D can do stop-down metering so you won't lose metering accuracy the way a Nikon D3x00 or D5x00 user of the 14/2.8 would. But you can only really use the Av or M shooting modes (i.e., modes where the camera doesn't adjust the lens's aperture setting). And you are also going to lose lens EXIF information (lens used, aperture setting used, etc.) It's not just autofocus you're going to lose. But not having to bother with the electronic communication protocols is how Samyang makes their lenses so inexpensively for so many different camera mounts.

The EF-S 10-18 f/3.5-5.6 is designed for an APS-C sensor, so if you move to a full-frame body, you will probably have to swap this lens out for another ultrawide. And it is slower, being only max. aperture f/3.5 at the 10mm end of the range, and f/5.6 at the 18mm end, so it's probably only useful outdoors, in good light, or on a tripod. But it zooms all the way out to 10mm, so it's substantially wider than the 14mm, and gives you a great deal more framing versatility. And it reports to the camera, so you'll have autofocus, lens EXIF, and wide-open metering, as well as access to all the camera shooting modes. It also takes 67mm screw-on filters.

Which lens is the better choice for you, depends, as I said, on your specific needs/preferences.

See also:

• Wide Angle Lens recommendation for canon 70D (cropped sensor camera)
• How do I choose a wide angle lens for my 700D?
• Does an ultra-wide lens perform well in landscape photography?