I see the BMCC has already been mentioned a good bit, but just for the guy who thought about upgrading from the mk3 to the c100, I would definitely take a look at this video:
Comparing the Cinema Camera & 5D Mk III on Vimeo
This puts the BMCC in perspective against the mk3, and it literally blows it out of the water. Now, I have nothing against the mk3 as I own one, but for video I would definitely take a closer look at the BMCC over the c100. You could get the BMCC, matte box, shoulder mount, SDD drives, an L lens, larger external display and a follow focus and still have change left over for the same price as just the c100 body. That, my friends, is tempting.
And this is just my opinion, but I'd still rather the Sony FS700 over the c100 from the looks of it. Same price range and the FS700 has a 4k firmware upgrade coming out.
It all honestly boils down to what you want though. People have made beautiful films dealing with the horrid H.264 compression in the 5Dmk2 (and nearly all other DSLRs). If you're serious about making movies, you'll find a way to work with whatever you get.
4k is marketing fluff. No one needs it. Have you every tried posting in 4k? Exactly. The c100 has a great sensor, awesome image processing, much less skew, an anti-alaising filter, everything but a solid codec, but an external recorder fixes that. An L lens does no good to make the BMC's sensor the right size or fix skew and aliasing...what l lens is wide enough to get you a usable FOV on the black magic camera? 18-85 is the range of every cinema package on super35 (zooms or 18, 25, 35, 50, 85 primes). That's equivalent to 12-55. So that's an 11-16mm zoom and a 17-55mm zoom and those are f2.8 on a tiny sensor, so shallow focus and low light suffer. There's no compelling prime kit and the UWA options are poor.
That video is obviously biased (the part on wide angle is particularly ludicrous), and consider how much additional work in post it takes working with raw footage. What if you shoot ten hours of that for an event video. How do you cope with that? 24Mbps has the advantage of recording a lot of footage--for doc or videography or wedding it's not so bad at all. Furthermore, apply HTP (which is only necessary in the tough shots that video focuses on) and sharpen the 5D footage in post and you've split the difference in that video at the expense of a bit more grain. But let's be realistic...the 5D is no video monster in the first place. Or just compare the C100 and BMC camera and I think you'll be surprised that the C100 outdoes it in everything except flexibility of the codec in post, again an easy fix and having that small codec can be nice for other gigs.
There's a lot of hype around buzzwords like 4k and raw, thanks mostly to Red. But 90% of new tv shows are shot on the Alexa (which can use raw, except no one is shooting in that format for tv, just for cinema). Likewise almost all content is posted in 2k or 1080p for tv or for cinema. And there are no consumer 4k tvs with any market share and 2k DCP is still the standard. If you can sell clients on 4k and raw, it's worth something, because clients are as ignorant as most people who buy into that kind of fluff...except they also are less likely to care about it (but some will!). But for actually getting good images to delivery the C100 is a lot more impressive than its specs and conservative design indicate, and I would take on over the BMC camera ten times out of ten EXCEPT in a studio setting or a greenscreen shoot or such.