This isn’t a photography forum, but you posted in rec.sport.unicycling, so I’ll assume you plan on shooting uni vids, and try to address that specifically.
Unless you either
a) come from photography background and are very comfortable using (D)SLRs or
b) are very serious about your videos and plan on spending a lot of time preparing and setting up the shot
I’d call a DSLR a poor choice for unicycling videos.
Rolling shutter and focus will be problematic. On the other hand, both issues are much reduced by using a wide-angle lens.
The major difference between the 5D and 7D is the sensor size. The 5D is much larger, so the depth of field at similar f-stops and focal lengths is much shallower. Note that the 7D sensor is very close to Super35 film frames (the kind most movies are made with).
So, if you want people to go “Oooh, aah” “wow, that looks so professional” “the image quality is amazing” mainly by virtue of your equipment, go for a VDSLR.
If you want to do really fast action, quick pans, and spend less time setting up, get a video camera with a smaller sensor. You can still get all those reactions, but you will have to spend the time setting up, and also probably carefully compose your shots, plan the lighting, etc.
Don’t forget: your footage will only be as nice as your lens, regardless of what camera you use. Get something nice.
And finally, to respond to a few other comments:
Overcranking. Note that this is not available on the 5D, which only shoots up to 30p.
Autofocus. The Panasonic GH1 (or whatever it’s called) does it, though it’s not technically a DSLR.
Handheld. Very difficult with these cameras, but if you want nice camera movement, you should probably invest in a steadicam anyway.
Depth of field. Almost any consumer video camera can get a reasonably shallow depth of field. The catch is, you need to zoom way in, and be as close as you can. For medium- and wide-angle shots, you need the bigger sensor.
Audio. The 1/8" headphone line in is fine, and appropriate for what the camera is, just get a good XLR adapter. The point where this is limiting is probably past the point where you would be recording externally. Hell, the video is compressed; if you really want quality get a RED.
My experience:
I just finished shooting about half of a feature-length film entirely on a 5D. I do a lot of photography with Canon DSLRs, so I only had to find the ‘record’ button to start shooting video with it, which saved a lot of time. It was wonderful not to have to fight the camera to get the depth of field shallow enough. That said, there were scenes where we wanted it deeper, and had to compromise between image quality loss (from high-ISO) and depth of field. I loved working with it, though.
