After struggling with the decision whether or not to get a Canon 100-400mm, I've decided that the 100-400mm would be too redundant with my Sigma 50-500mm. So, I'm looking at a telephoto prime instead.
I've got the Canon 1.4x and 2x teleconverter and a Tamron 1.4x. I like the reach of the 400mm but also like the IS on the 300mm.
Does either of these lenses stand out above the other? How about when you add the teleconverter?