Since you are planning on using flash (based on having answered your flash question) either lens will be suitable since both are an f2.8 constant aperture. It just depends on how wide you want to go.
Just remember to apply the 1.6 "crop factor" when considering either lens. You'll also need to review the wide angle coverage of the Canon Speedlite 430EX II previously suggested.
If you weren't planning on using flash I'd have to go with the Canon EF 50mm f1.4 since it is a full two stops faster than the two zooms. In fact, I'd still suggest having it in your camera bag. If you decided to shoot solely with available light, two stops is significant. If conditions using ISO 800 gave you a shutter speed of 1/100 sec. with the f1.4, you'd have to use ISO 3200 to get the same shutter speed with an f2.8 lens.