I hate to choose "which lens to attach my 7dii (or 5diii) now, and which other lenses to take with me too"... I'm using 400mm f5.6L mainly for BIF. I use 300mm f4L IS for butterflies, dragonflies, snakes, birds closer to me (ok also for BIF), and if I need f4 and/or IS. Tamron 150-600mm when I need 600mm for static shots. I've thought many times, I should sell all these lenses and buy 100-400 ii + 1.4x iii... I know 100-400mm ii is great lens. But I'm hesitating because while shooting, I love the weight, size and handling of these primes. I also love their built-in lens hoods. 100-400 ii would be heavier and has that plastic lens hood. I would also loose f4 at 300mm and f6.3 at 600mm. I'd like to ask, did anyone of you regretted after doing this switch??