I had both and took both last August to Tuscany - including the places you mention. The primary purpose of a TS lens is keeping your verticals straight. In architecture for most shots if your verticals are not straight, then others will not bother looking at your work. They're really a basic necessity type of thing. Some may argue that you can do this in PP, but that is not true - the perspective will be different.
IMHO it's not really a one or the other proposition. I found both necessary but if I had to take only one I would take the 24 because 17 is often too wide and it has more problems with flare.
I posted a thread here recently with examples from Tuscany with these lenses - http://www.canonrumors.com/forum/index.php?topic=13128.0;topicseen
Very helpful pictures - thank you!