I'm currently shooting with a 1DX and 7D2, my go to lens for birds and wildlife is the 600 IS II (with Mark III extenders when needed) and I also currently have a 300mm 2.8 IS and 100-400 II in my stable as well. The vast majority of my shooting is done on the 1DX. Frankly I haven't been all that happy with the 7D2, it's a great camera but it's the first crop sensor that I have owned in quite a while and the IQ just feels lacking to me compared to what I've grown accustomed to shooting FF. That said it does well enough in good light and is nice to have when I absolutely need the pixel density, and I did like having it mounted on the 100-400 during soccer season. Anyway, the 100-400 is the most recent acquisition and I bought it with the idea that I would try it out and then decide whether I wanted to keep it or the 300 2.8. I'm leaning toward keeping the zoom and selling the 300, mostly because of the increased flexibility and ease of carrying when I am hauling it around as a secondary lens with the 600. I love the 300 and I'm struggling with letting it go but being honest I think in most situations I would reach for the zoom first and the 300 would be left sitting.

My original thought was to sell the 300 and the 7D2 and pick up a 5DIV. The 5D is appealing to me for the 30MP sensor. I've never felt like I get the full benefit of the 1.6 crop from the 7D2 because of the image quality compared to the 1DX, so I feel like going to 30MP on a full frame sensor would get me back in the ball park as far as effective reach plus giving all the other benefits of the new sensor when subjects are closer filling the frame. I wish it was 8fps instead of 7, but I got by just fine with 5fps from a 1DsIII for quite a while so I'm sure I would manage and I would obviously still have the 1DX when I need the frame rate.

It recently occurred to me though that another option would be to sell the 300 and the 1DX and grab a 1DXII. This would leave me with the 7D2 as my 2nd body, but... 1DXII. I wish the 1DXII had come in at 24MP, I wouldn't even have to ask then. I'd like to hear some thoughts from people who have hands on experience with both the 1DXII and 5DIV. I guess it's a question of whether I raise my floor by improving on the "weak link" in my lineup, or raise the ceiling and improve on the body that I do 90% of my shooting with already. I will say that the thought of getting a 1DXII is making the idea of parting with my 300 more palatable, but not having any experience yet with either the 1DXII or 5DIV I'm really just theorycrafting the pros and cons. I'd like to know what everyone else thinks.