I've used both.
I took the XR550 in a Light & Motion Bluefin housing to Cayman and got some great footage with it. Its a nice package if you can afford it. You get lots of recording time on the 550, and a super easy, one touch white balance on the Light & Motion housings. The problem is the Bluefin housing is expensive, and the Fathom 90 lens, which I consider essential when shooting, is just as expensive
I've just purchased a Canon 7d in a Nauticam housing. The 7d does outstanding video, and shoots 1080 at 30fps. The best part of this setup is interchangeable lenses - something you can't get on a consumer/recreational level camcorder. That's worth it's weight in gold right there. The Nauticam housing is super ergonomic, and easy to use.
The downside of the 7d - the white balance process is a bit more cumbersome, and is certainly not 'one touch.' The other major drawback, if you're using if for video is the memory cards - you need to use a super fast one. Also, if you record too much, the card will heat up, and a little thermostat icon will appear in the LCD. The card does physically get hot.
Overall IMO, the 7d is the way to go. You get the best of both worlds with HD video and dSLR. The ability to change lenses puts the 7d over the top. Film students are buying these en-mass for their projects, and the Canon 5d mk II was used in the filming of the TV show House, and the movie Iron Man II.