Can any of you tell me which would one be preferable for landscape photography?

The slim one is slimmer, which means that there's less chance of the edge of the filter being visible in images taken on a wide angle lens. An 11-16mm lens is certainly wide enough to justify a slim filter on a full frame camera, but since your 550D has an APS-C sized sensor, you're not going to see the edges of the image anyway. Unless you plan on trading up to a full frame camera at some point, either the slim or the non-slim version would work for you.

If you've never worked with one before, you might not realize that a circular polarizer is actually made up of two pieces that rotate relative to each other. This lets you change the orientation of the polarizer, which varies the polarizing effect. A regular CPL will be a couple millimeters thicker than the slim version, which means that you have a little more to grasp as you operate the filter. So, depending on your hands, you might find the regular version a little easier to use.

You should know that polarizing filters and very wide lenses can make the sky look blotchy. On a normal or long lens, CPL can help you get gorgeous deep blue skies, but the effect depends on the direction you point the camera and the orientation of the filter. A wide angle lens sees light from many different directions, so the polarizer's effect varies across the frame and can make the sky look uneven -- light blue in one direction, dark blue in another. If you know about the effect you can always choose to remove the filter when it would be a problem, but it can be an unhappy surprise if you forget.

Finally, buy the size that'll fit your largest diameter lens and get step rings that'll let you use the same filter on smaller diameter lenses. Step rings weigh almost nothing, so their easy to carry, and they're a lot cheaper than buying several versions of the same filter.