You don't list the lenses you have, so I surmise that you're planning to rent a lens for the occasion. Under that assumption, I think I'd go with the EF 16-35mm f/2.8. It's a lens I've never used, but it has a good reputation. (I think there are actually two versions, so go with the more recent one if possible.) I've gone as high as ISO 1000 on my 50D with acceptable results, so I'm guessing that you'll be OK thus equipped.
The 17-55mm f/2.8 IS is also very good and almost as wide. (My wife and I both own that one. If you rent or buy it, check the AFMA before using it in a critical shoot. I didn't, and the shoot was a fiasco.)
The choice is in a Canon is the EF-S 17-55mm f/2.8 IS USM Lens. Better yet, in a Sigma is the 18-35mm F1.8 DC HSM. This is as fast a lens as you can get in a zoom of this range. I prefer the Canon but the very good Siggy is more than a stop faster and IQ is just as good.
You do not want a prime especially not knowing the space.