First off, nice flash and lens. I have both of those (with the 20D) and that is a capable package.
What mode you want to shoot in is really dependent on what type of shot you are trying to take.
It is really all about the effects you get when you change the aperture or the shutter speed. Its also partially about what metering mode you are in.
The larger the fstop (smaller the aperture) the more depth of field you'll have. If you want separation between the subjects and the background, then you want a smaller fstop (smaller depth of field.) If you want more depth of field (getting the entire group of people in focus) then you want a larger aperture.
If you want to stop action, then you want a faster shutter speed. But to get a faster shutter speed you either need lots of light or a lower fstop. This means that you trade stopping action vs. larger depth of field. Get it? There is an interaction there that you have to learn if you want to get exactly the effect you want.
Does all this make sense?
To make things more interesting, you can also vary ISO. If you increase the ISO, you double the shutter speed. but you can also keep the shutter speed the same and increase the fstop 1 hole stop (increasing depth of field.) Of course, doing this increases the noise. If you find the noise acceptable for a given ISO, then you're all set.
The last point is metering mode. Learn about how the metering modes work. This is an important thing because some metering modes are better at specific scenes than others. Very back lit scense are better with a spot meter (well, Canon's equivalent to the spot.) That will allow you to meter off exactly where you want properly exposed. But a center weight average is better when you have several subjects (all in the center) that you want properly exposed. Evaluative works by metering the entire scene evenly. What people usually do is learn one metering mode... learn when its right and when its wrong. And then use exposure compensation to correct its flaws.
Does that help? Not the "do this for this, do that for that" answer you might have been looking for... but photography is not that way.