I currently own a Sony EX1R and Canon 5D Mark II with a wide assortment of lenses. I really like the quality and film look of the footage I get from my 5D, so much so that it has become my main camera. I shoot mostly weddings and lots of sports - should I sell my EX1 and purchase another 5D? Maybe a C100? Curious to see what people think.
I made a similar decision and never looked back, however there are now certain jobs that simply are not very practical for me to do - mainly those that require long continuous filming times - but for me such assignments don't tend to be the interesting jobs anyways.
I wouldn't be shy about using a Canon 7D or the full frame 60D as my second camera - there's so much other gear that you will need to outfit your second camera that I'd save money where I could. I doubt very much your clients will ever notice the difference between these cameras and a 5D M II or III.
I went with the c100. It probably gives you everything that the Sony does except a rack zoom. But for the money, it's a fabulous all purpose video camera that gives you great DOF when needed. I still have my camcorder around for certain gigs. But I now use the C100 more than my 5D mkiii. It's now all between stills and video.