I have used them both for exactly what you are attempting. I shoot wildlife documentaries. I bought the bmcc for the crop sensor primarily and of course the 2.5k raw was a nice perk. After using it for a few days I realized its quite different. I had been using DSLRs for a few years prior. The camera is awkward to use and the added things like batteries make it even heavier and more awkward. The image quality was really nice and the 2.3 crop was really awesome for wildlife though. Low light was average at best. I felt like it was pretty close to a 7D and not as good as the 5d2 which is what I had at the time. Im now using 5d3's and filming raw because of the Magic Lantern Hack. The 3x crop mode holds up extremely well and looks just as good as the full frame stuff. Low light is far superior and the camera is much easier to use (ergonomics and accessories like batteries). Not to mention weather sealed to withstand some serious elements which I run into often. Also of course you are holding onto an excellent photography camera which is a great bonus.
Pros for 5d3 in my opinion:
- Easier to use
- More crop when wanted/needed
- Photos
- Weather Proofing
- Lighter
- Low Light Performance
Pros for 5d3 in my opinion:
- Easier to use
- More crop when wanted/needed
- Photos
- Weather Proofing
- Lighter
- Low Light Performance