I have experience with the DSLR's.
What many are doing is recording the audio on the camera with the internal mic, and then using a real microphone hooked up to an audio recorder (such as a Zoom H4). The 2 audio sources are then synced using an application called "Plural Eyes" in Final Cut. That is where my brackets come in (see my link below or go to
The J-Rod & J-Cube Microphone Mounts and Professional Camera Accessories). The J-Cube hold an audio recorder, a mic, and a wireless mic receiver all together.
In terms of camera choice I won the 5DII and love it. A colleague of mine uses the 5D and a 7D and intermixes the footage without any problems. The 7D is a crop chip camera, so your lenses will be multiplied. I want to add another body, and have struggled with the decision to get another 5D or add a 7D. The 7D does focus much faster in video mode, and shoots twice as fast in still mode, and costs less. But I would like 2 bodies that are the same ergonomically.
Please remember to check out my link below. With The J-Cube and a Pro Shoe you can mount an audio recorder and other accessories without the need for a large cage and keep your camera manageable for handheld shooting.