The difference between 300 and 400 is indeed, not that huge, but when you put a 1.4x TC on for example suddenly your looking at 420 vs 560mm. If you use just a 300, then 420 is about the most reach you can get with reasonable sharpness. With a 400 you can get that 400mm reach already and at a stop faster and then add a TC and get up to 560mm. You then need to use a 2x TC on the 300 to match the 400 with just a 1.4x and of course a lens with a 1.4 is much better than a 2x......
Bottom line is that you should buy the lens that gives you the reach you need without needing to resort to a TC, and if you do sometimes need a TC, using a 1.4x is always better than a 2x