Cotton/poly is the most common. The reason I've heard from manufacturers is that it is the best compromise between durability, lack of shrinkage and flexibility. 100% cotton is usually more flexible and easier to stretch but at the expense of possibly having to restretch it again later.
Besides composition, the other variables are thickness, surface texture and whether it contains OBAs.
From what I gather from the various printing forums, the most commonly used canvas is Breathing Color's Lyve (no OBAs). It is a matte canvas that offers a good combination of price, a relatively smooth texture, and a high quality image. It is also one of the few canvases that can be used with either PK or MK ink and still look good (I use it with PK). Both B&H and Adorama carry it.
The smoothest surface is provided by Canson's Museum ProCanvas, which is 100% cotton and OBA free. It comes in either matte or satin and is very soft and easy to stretch. It is carried by I.T. Supplies and Adorama.
The canvas with the whitest background (without OBAs) is Premier Art's Museum Bright Satin canvas:
http://www.premierimagingproducts.com/museum-bright-satin-canvas/Supposedly the whiter the coating the better the colors, but I haven't seen much difference compared to either Lyve or Museum ProCanvas. It's also slightly more expensive than the others. This canvas is available from I.T Supplies but I'm not sure if B&H or Adorama carry it.
All canvas (and paper for that matter) should be coated, even those that say you don't really need to. This is because the microporous inkjet coating has lots of tiny holes in it so the ink can be absorbed. But those holes need to be plugged once printing is done to keep air pollutants from attacking the ink. Paper prints framed behind glass are for the most part protected by the glass, but any print exposed to the air, like canvas, needs to be protected.
Stretching is the final component to consider. You can either stretch it the conventional way using standard stretcher bars and pliers or you can use a kit.
I opted for the kit as it's just easier to get good results (once you've figured out what you're doing). GoFrame in Canada makes all the kits sold in the U.S. that I.T. Supplies, Breathing Color and Lexjet resale under their own store brand. There are two lines, the thin and cheaper amateur line and the thicker pro line. Unless you only want to frame a small canvas stick with the pro line as the cheaper line will warp past 11x14.