thanks everybody, this helped...
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This is the textbook example of what the TS-E 17 is designed for: close-up photos of tall buildings.
Of course, that's not all it can do, and it's not all it can do really, really, well. But it's basically its primary purpose.
If you're thinking of doing this sort of thing -- or of similar sorts of close-up shots of very big things (including mountains and trees) where you don't want any geometric distortion -- then this is the lens for you. If you have other types of photography in mind, this probably isn't the lens for you.
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
What Canon lists their price and and what actually gets charged is completely different.
I don't know about the US, but in Germany the street retail price of current Canon products unfortunately is near the list price, maybe a couple of €100 or so off depending on how shady the dealer is - but certainly not "completely different" :-o