The 300mm f/4 IS at 300mm with no 1.4X gives you a full stop more aperture than the zoom. But as soon as you put the 1.4X on, the zoom pulls ahead in image quality (and focus speed and bokeh, I bet):


ISO 12233 comparison of 300mm f/4 IS + 1.4X and 100-400 at 400mm


Therefore, if you plan to use a focal length of 300mm the most, and very rarely, if ever, use 400mm, then I would recommend the 300mm prime. Or, if it's more importnat to have f/4 aperture (and you'll live with shorter focal length to get it), I would again recommend the prime.


But for all other purposes I would recommend the zoom.