Reasons for using DNxHD instead of ProRes?
I have used both on my Ninja Blade with my C100, no real difference in quality across drastically different circumstances (night vs day). Pro Res works better on Mac, DNXHD works better on Windows. It all comes down to your OS, and whether you have to share the files with broadcasters (in which case I would use DNX as a lot of editors use Avid).

Other then that, I haven't heard, or seen first hand any differences between the two and I can honestly say that any differences pointed out online are mostly conjecture.