I can't speak to the specific lenses you listed, but if you are asking whether to get a f2.8 or an f4 lens, I say go with 2.8.
You can always buy the f2.8 lens and set the aperture to f4 if you want deeper Depth of field. But if you buy the f/4 that is the most light and shallowest Depth of Field you can get, no matter what the situation calls for.
For my money, I don't buy a lens slower than 3.5. You can always go up in f-stop but you can only go down as far as the lens will allow.
Now as far as Canon vs. Sigma and 17-50 vs. 24-105, there are many other issues and things to consider, but the answer to your f-stop question is a resounding "go with the 2.8", (in my opinion).