I am shooting with a 550D ( T2i ) .
when stormchasing, I prefer to shoot in FHD 1080 at 24, 25 or 30i/s , because the HD 720 will shoot at 50 or 60 i/s and that is less favourable in low light situations. indeed, you win one f-stop .
BTW, shooting 1 minute in FHD1080 or HD720 gives about the same filesize.
I recommend you get the extra battery grip, and a 2nd battery, because shooting video is only possible via the LCD screen and that uses up a lot of power. Ideally, you even keep a 3rd battery in the pocket.
Ideal lens today is the EF-S 15-85 IS . It is an excellent lens, see the reviews on fredmiranda and B&H, and its IS Image Stabilization works very very good in video mode. The 15mm results in equiv 24mm , nice wide angle. Other lens I like is EF-S 10-22 but it is not stabilized and this is easily seen when shooting handheld, even at 10mm superwide. The 15-85 is actually from Canon the widest angle lens with IS. All wider lenses have no IS. Thus, if you want a wide lens with IS, this is your best choice.
Vignetting is visible on the 15-85 lens especially wide open in wide angle. However, the camera has a setting in the menu function to compensate for the vignetting and it disappears almost entirely.
outside microphone : would certainly give better sound , and you can reduce wind noise , and get a stereo mic, etc. but using an outside mic makes the camera more bulky, and you have to keep an eye on the microphone's battery level. In my opinion it is not a must. Stormvideo is most spectacular from what you see, less from what you hear, in most cases.
here are a few videos I shot with the 550D / T2i
in this one I used either the 10-22 lens, or occasionnally a 50/1.4 . None of them has stabilisation.
As you can see, handheld without image stabi , shaking is visible, even at superwide angle. It is important to keep camera very calm. if you can, use a tripod, or at least a monopod, or put the camera on a stable surface.
http://www.youtube.com/klipsigeneva#p/u/16/HVAStCQBFdI
http://www.youtube.com/klipsigeneva#p/u/15/qKfFZ1oqcNk
originally filmed in 1080FHD , then reduced to 720 HD and slightly compressed for faster upload .
next : filmed with the 15-85 lens WITH image stabilization ! Makes a big difference.
http://www.youtube.com/klipsigeneva#p/u/12/WyfzulWAJA0
http://www.youtube.com/klipsigeneva#p/u/10/WgBjdPdpogk
http://www.youtube.com/klipsigeneva#p/u/5/E_o37NVxy2o
http://www.youtube.com/klipsigeneva#p/u/0/k28D_0ERgJE