ddk632 is somewhat correct and as others have pointed out, there are real consideration beyond camera model that may steer you toward a dedicated video device.
There are also some things that you should be aware of concerning shooting video and these likly apply to any DSLR made by Canon. One major consideration is based on public law, not camera capability.
The manual for most Canon DSLR's mentions that video recording can shut down if the sensor overheats. From my understanding that typically only happens after 15-20 minutes but could be sooner if you are repeatedly recording videos, even in segments. Not that it likely has any bearing on your question, at least at present, but when you make your decision use a class 10 card if at all possible and keep in mind that there are a tremendous number of counterfeit cards in the on-line marketplace. They may be labeled 10 but could be something altogether different. Get your memory from a reputable source, read that not eBay!
Also, be aware there are some limitations on how much you can record, even with the best of memory cards. More than 12 minutes can be recorded but there are some limitations. When the file size reaches 4 GB on most Canon DSLR's, recording will automatically stop. That means that at 1920x1080 dpi and 1280x720 dpi the maximum recording time of a single movie clip is about 12 minutes. At 640x480 dpi it is about 24 minutes. Check the 5DMKII (or 5DMKIII) manual.
These numbers may vary slightly for the 5DII, but capacity of the card differs in recorded time; at 1920x1080 dpi or 1280x720 dpi you can stuff about 44 minutes total of video onto a 16 gig card. At 640x480 dpi or crop 640x480 dpi this goes up to 1 hour 32 minutes. However, from my 60D manual, "the maximum recording time of one movie clip is 29 min. 59 sec. Depending on the subject and the increase in the camera's internal temperature, the movie shooting might stop sooner than 29 min. 59 sec." The 29 min 59 second limit is for legal reasons. If the camera could exceed that time limit, then it is classed as a video device and the import duties would change. This is not just the Canon line, Pentax for instance states "Like competing DSLRs, the Pentax K-5 also monitors sensor temperature during recording, and will halt capture if the temperature rises beyond a certain threshold." A reputable source also states "This limitation is due to the different (European) import duty rates for still and video cameras." There is no reference made to differing duty rates of product entering the United States however. So there may really be two limitations, one, the 29 min 59 seconds limitation imposed legally, and a sensor temperature issue that might also be reached in some rare circumstance.
By the way, the shutdown due to temperature can be a real drawback so consider purchasing a true video device if you are capturing something critical. At a recent horse show I was recording almost continuously and my 60D stopped based on sensor temperature. I had to wait several minutes before I could restart, missing several horses in the competition.
5D Mark II
