This really depends on a couple of things. Cost first…are you willing to spend for either the 400/2.8 or the 600TC, that’s the biggest question and only you know that answer. Second…what focal lengths will you want to use more often? Of 400 and 560…get the 400, if 600 and longer get the 600TC. Back to the first question of the expensive on either dollars or weight (an important consideration). Next…are you making money or amateur, and where does the output go. If primarily screen…output is going to be downsampled and a lot of the ‘better’ disappears in that situation…not all but enough that maybe that tips the cost balance. Them go for most use lengths…for me here in SW FL the 600PF and TC provides 840 or 1200 in a pinch in good light and that’s fine…I decided the 800PF was too limiting in both size and weight and flexibility for me. I also have the 400/4.5 and the 100-400 and generally take the latter on my second body. Brad Hill has a very recent post on the 7 ways to get 800mm-ish and ranks them according to sharpness and other factors, as well as Steve’s videos on the various comparisons. For me…at going on 70…size and weight are the top criteria because we hike with the gear…and while we could afford the exotics the size and weight are such that they would likely be about the only lens I had on the hike and that loses flexibility. Second criteria is bang for the buck…and with almost exclusively screen output the ‘this one is slightly sharper at 200%’ evaluations aren’t much help…because better is still the enemy of good enough. OTOH…if my wife were to wake up one morning and decide that being my Sherpa was an OK thing…I might buy the 800PF but that’s about my limit in the price vs everything else comparison, at least until I win the Powerball and then I can hire a Sherpa. Edited to add…with todays NR software and better sensor tech the loss of even a stop isn’t nearly as big a deal as it used to be, and while I admit the bokeh of 2.8 or 4 is nice…the new BG blur feature in LR can if used correctly and sparingly increase the bokeh some…not as good as a rel 2.8 of course but an improvement yes. The whole thing comes down to your needs, budget, and what compromises you are willing to live with because every solution has compromises.