I sold my 100-400 for a 300/4 IS. I decided to get this as the wildlife pics I go for are bugs, dragonflies, butterflies etc. The IS was a requirement for me. I know that I always used my 100-400 at 400mm, so was torn between the 400/5.6 and the 300/4 IS. The real clincher for me between these was the minimum focusing distance. It is really quite far on the 400mm, which would not have suited my particular wildlife needs.
So, I can add the 1.4x to get 420mm. It does slow down the AF a tad, but I can live with that. I also use extension tubes at times, so I can get even closer to the bugs. I never regret changing the 100-400 for the 300mm.