You might consider the resale value of each. If you buy new, the lower the price of the camera, the less you will lose when you resell it (at least in absolute terms). If you buy used... you might not lose much at all if you resell it.
I tend to think of all my buying of cameras and lenses as "renting" them. I'm always ready to sell them if need be. Plus, when you rent you deposit the value of the camera/lens anyway.
I think most of us can relate to beating ourselves up over what to purchase, but from my own experience I can say that I'm never 100% it will work until I actually use it.
It's a work purchase, unlikely to be resold, just run until it breaks or is obselete. If I was using my personal cash for it, yep absolutely, it's going to be bought to be used though, and with no mind to resale.
Assuming you have the budget for a new C100, I would get a C100 for bread/butter shooting. As I see it, the 5D3 and the Blackmagic cameras have a lot in common but they each have their inconveniences and extra work or gear required.
I've only shot with the 5D3, but I can't imagine the Blackmagic cameras are that much easier to use (either during shooting or in post). On the other hand, I can see how a C100 would be much easier to deal with in terms of shooting and workflow. Ultimately, I feel like it's a question of shooting needs vs quality needs. The trade-off would seem to be between ease-of-use vs ultimate image quality. If I was shooting run-and-gun solo stuff and 1080p AVCHD was good enough... bam, there it is: C100. But if you're gonna be tearing it apart in post and shooting for maximum quality: 5D3 or BM.
Personally, I could never spend that much money and still not have something better than 1080p AVCHD. But if I were a pro shooter, I would purchase the one that was right for my work. Either image quality is important, or it's not. In this case, it doesn't sound like it's that crucial (if it were, you would have stated the minimum quality needed). So it sounds like C100 is the way to go, to save you time and headaches for image quality you don't need.