Go with what you found great the 100-400 zoom. There is no difference in f stop compared to the prime, you get more flexibility, IS and thus better resell price once you would opt for a big white. I went for the 70-300 L which is a far more modern lens and will give you 480mm on a crop. Also I find the 70-200 L plus 2xextender a very useful option but of course also more expensive. Good luck choosing.