I would go for the 2nd hand Canon L 17-40 because of it's track record, build quality, greater re-sale value and the bottom line factor: top-notch repair and after-sales service for it unlike ehem, some brands where all you get is some crap excuse and everything also have to 'send back to Japan, have to send back to Germany bla bla bla'.
If you need more reach, flip freak suggested a Sigma 18-200 but the wide end suffers a bit, something that may bug you to no end for travel applications like landscapes and especially architecture.
Actually, coming to think about it, 17mm is also no great shakes - it equates a FOV of a 28mm lens on a full frame when used on your 30D, so it a little wide, but not that wide. Personally, I would find 17mm too restrictive, but that's just me.
Or you could get adventerous and go with a Sigma 10-20mm and a cheap 50mm prime (or a top-notch, super affordable, premium quality manual focus 50mm prime). Not a bad idea as it might force you to see things a lot more creatively.
A lot of these things depend on your personal vision and ways of approaching a given subject.