I welcome some advice on what to buy, the 100-400/4.5-5.6 or the 300/4 + 1.4xTC. I will use it mostly for birding. If I get the zoom, I will use it mostly for birding in the long end of the range. I have the 1.4xTC already and will need to pay almost the same amount of money for either. I really appreciate your advice.