Kathreen, I’ve owned both cameras, still own the C100, and can say that they are very different cameras. It really comes down to your budget, and what you want to achieve in the look of your video. The C100 requires extra lenses. The xf100 comes with a decent but ‘short’ lens, and does allow zooming, which is really valuable in shooting theatrical and other live performances. If you need to add a second camera, the xf100 is also cheaper. It does have some downsides, however, in that it cannot shoot in as low light as the c100. Hopefully your theatrical work will be well lit. But I prefer my C100 in all situations. If you can afford it, it’s worth it.
What I might recommend to you is to look at the XA25 or XA20 from Canon. It has a lens that has a longer zoom (20x) and believe me, that is important in theatrical at times. It also has a slightly better sensor, so it is likely to do better in low light. I shot with one last month, and it was better than my xf100 was by a small amount. It’s also smaller, and less noticeable for documentary type of work.
Another option is to look at the new XF200 or 205. It also has a better zoom and other features from the xf100.
I mention the 25 or 205 models because if you need to add second cameras, you may want to link them together. These models come with more external options for doing that, such as SDI out. If you want to save some money, just by the xa20 or 200 models, and worry about connectivity later.
While neither of these cameras are as good as the C100, they will save you a lot of money, and give you plenty of great features while you learn if you really want to invest more in video. You can always sell them or use them as a second camera if you step up to the C100 later.
good luck.
Al