I have a few weddings coming up in August, and my 550D is just not cutting it for the lowlight. Still saving up for the C100 for my HMC40 replacement.
Anyhow, I was going to rent the C100, but the rental is pretty high compared to renting the 5d mark iii.
Question is, I'm used to shooting the 550d with ML. How much of a learning curve is there to use the 5d mark iii for video coming from the 550d? Will one/two days be enough to prep me for it's use?

The biggest difference will be button locations (for me, how to change your aperture was the biggest change).

Much better white balance control options on the mark iii.

Moving over to full frame will probably through you for a loop, too, so make sure to keep that in mind as you choose lenses. The 135mm that got me a nice, tight shot for vows on the T3i is too far back on the full frames, and need to use about 200mm to get the same effect.

Ok, thanks. Have a tamron 24-70 and 70-200 to use with it, since the Sigma 30 won't work. I'll give myself a day or two to acclimate.

At least get a Canon 50mm f/1.8 for $100. You'll want that f/stop at any reception in the low light (same as 35mm on your old 550D), and I use it when I walk around shooting faces - a little bit of distance but picks things up well.

You don't need to use an F/1.8 lens for low light on the 5D3 as you can crank up the ISO to 12800. If your zooms are F/2.8 then you will be fine. The DoF of the full frame sensor is really shallow at wide apertures & one of the beauties of using the 5D3 is that you can shoot at F/4 or even F/5.6 & adjust the ISO to get correct exposure without fear of noise.

Good to know, Nigel. We don't go above 6400 on our Mark ii (we had a *REALLY* dark reception) and don't even pretend to use our 600D/T3i's once the dim of the reception turns into the dark of the dance.