Here are my thoughts about this. T3i is a good camera, but my experience with shooting interviews, public speakers and similar shots tells me that you may have some trouble with a fairly large-sensor camera. My biggest problem that I consistently had while shooting interview and meetings is the ability to quickly and accurately focus. Auto-focus on all Rebel cameras is not really usable for video (these are still photo cameras, after all). That means that you would need a very sharp eye and extremely light touch in order to constantly ride that focus ring. I have 5DmkII, and as soon as I touch focus ring, the camera moves and the shot is useless. After a lot of practice, I have learned how to do it without disturbing the lens, but this is quite difficult and severely constrains your movement and framing options. While follow-focus kits help a little, they don't help that much. More importantly, for these specific types of shots, shallow depth of field is a serious problem. In most situations, I need my entire frame in focus, and this is often extremely difficult, or even impossible, with such large sensor.

Long story short: DSLR cameras are amazing for narrative filmmaking. Actors are trained to hit their marks, so the focus is set for specific distance. When they get anything wrong, we reset and do a few more takes. You can take extra time to get the shots right, if you got the focus or exposure wrong. With documentary filmmaking, there are usually no extra takes, and you must get it right on the first attempt. This is why proper video camcorders tend to work better for such purpose.

As for using Vixia HF M-series, these are pretty good consumer-level camcorders (I have one, but an older generation). Since they are cheap consumer devices, the result will be nowhere near as good as from T3i. With careful lighting and preparation, the image quality of Vixia may approach that of Rebel, but you will likely struggle with different types of limitations with a consumer camcorder. More importantly, it will be rather difficult to match the footage shot with such different devices.

I would likely spend my money on something like Panasonic HMC-150 (if you can afford it), or HMC-80, or HMC-40).

The light kit is very cheap and rather feeble (just 45W CFL makes it approximately like 200W tungsten, which is not much). You may be able to light a person's face or torso, but anything beyond 2-3m distance and the lights won't really give you much. Within your budget, though, you won't really find anything better.

Thanks for the input. Someone turned me onto the Panasonic Lumix GH2, which is a DSLR that has silent continuous autofocus, and unlimited record time (which is useful for an interview situation...the 10 minute time limit on the t3i presents problems).

I have looked at some interviews done with a GH2, and the video is extremely good...I would say definitely better than the cameras used to do interviews on T.V., anyway. The hacked version looked downright cinematic...as good to my eyes as a Hollywood film..