Canon 24-70L or the 24-105L?

I am about to ask the infamous question...should I buy the 24-70L or the 24-105L? It will be used on my Rebel XT as my main "go to" lens. I like the f2.8 of the 24-70, but like the IS and longer focal length of the 24-105. Is the f/4 of the 24-105 suitable for indoor use? How often do you really open up to f/2.8 for indoor shots? Doesn't that mess up the DOF, so wouldn't you be stopping down and not utilizing the f/2.8? As my "go to" lens, I feel that the longer focal length and the IS will be most useful, but I would also like to own an f/2.8.