Keep in mind: 1 The non-L Canon 70-300 has just micro usm, so you'll be better off with a 70-200/4L no to speak of the better build quality. 2. The depth of field with 300mm is very thin, so imho f2.8 is great for fast af but not for the actual shot, I even stop down my 70-300L a bit in good light.
Given all that I think it's gonna come down to (keeping the 135 f2, of course) the new 2.8 Tamron or the f4 IS Canon.
