If the lens is going to be just for the sake of having a telephoto for when you need it, get the 55-250. You're not spending very much money, and you're getting a great lens when you do need telephoto.
On the other hand, if you plan to use it more, then you have to see what you plan on using it for. The tamron 70-300 vc is actually a bit heavier than the canon 70-200 f4, but its more compact, so could be better for portability. The canon is sharper, there's no contest there, but also costs more. Personally, coming from the 55-250 I didn't feel the tamron brought enough of an upgrade in terms of IQ to justify getting it, so I went for the 70-200 f4. Now when I need something very light and portable, I have my 55-250, and when I know I'm going to be shooting something that requires telephoto and fast AF, I have my 70-200. I think the tamron is more of a general purpose telephoto, whereas the 70-200 is well suited for portraits and landscape.