The difference is that linen is made from flax fiber (a certain type of plant also used for its oil [flax seed oil]), often linen is also made from cotton which is actually a misuse of the term but has become common practice, and canvas is made from hemp. It is not true that the stength comes from the epoxy. Without the fiber, the epoxy would be a lot weaker and without the epoxy the fiber would be simply a sheet of linen (canvas). In general several parameters increase the strength of the fiber reinforced material: the higher the fiber content WITHOUT the materia breaking up (so there is obviously an optimum content for each fiber), tensil strength of the fiber, directionality (woven fiber instead of stirred in fiber like in FRN) and hence length of the fiber.
As far as I know, tensile strength of the fiber/wooven product is slightly higher for hemp than for flax and it is most certainly higher than for cotton, so canvas mircarta should be slightly stronger. For an application like a knife handle, which, in comparison to other possible uses of the material is pretty low strain, I would be surprised if you would see any difference in strength between canvas and linen mircarta in actual use. Since the linen is finer, the linen mircarta has a slightly finer appearance and texture, but not quite as fine as paper mircarta.
Hope that helps.
