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?

Which 300 and which 400? Canon makes several of each.

I do have the 300L/4 IS and the 400L/5.6, as well as the both Canon TCs. They're both great lenses, but image stabilization is a huge benefit for me, I use the 300 (often with the 1.4x) a lot more often because of that. Much of the end of last year I was working in conditions that precluded the effective use of a tripod, which may not be your situation.

--Joe

The 300mm f4IS and 400mm f5.6.

IS is something in my mind that's nice to have but not an absolute requirement. This lens will be primarily used for birds and occassionally little league baseball. It will be a mixed bag of shooting supported with a monopod and shooting unsupported.

Another lens that has come into play is the Sigma 120-300mm f2.8. It's an economical way to get a 300mm f2.8 but how does it rank with the Canon 300mm f4 or 400mm f5.6?

Went with the 400mm f5.6L. Going with the longer, natural 400mm made more sense and this lens is tack sharp. The 300mm f4L IS is not shappy either but IQ does falter a tad when you put the 1.4x on it.

It just made more sense to get the sharpest 400mm set up as possible.