Anjin, go back to the photos of the bird of paradise that I posted in this forum. There is a definite difference to the 100-400 with 1.4TC vs the 180-600. I wish, that day, I had thought of also throwing in a like photo of the 100-400 in DX mode so that we would see a 600mm to 600mm comparison. If you're not using 600-900mm lengths, then you don't need the 180-600. BUT...if you need that once in a while, the 180-600 is perfect in your kit. I don't use my 800 prime often but when I need it, it's nice to have. Only the 800 prime is almost $5K more than the 180-600 so it's only a matter of money and your use case. If you have something that you can use that can be bumped up to 600mm and you don't use it much, I think your answer is obvious. If you need longer and the price doesn't break the bank, then the 180-600 works well. My two cents but listen to your inner self. As my wife tells me often on matters like this, "think with your brain, not your heart." Want is a heart decision. Need is a brain decision. And sometimes if it's worth it to you, you just go with it and get the darned thing. Good luck on a decision.