Both lenses should be very fun, and both usable in their own ways, but very very different. The 17-50 is very much a 'normal' lens - wider than what you have, but not really real wide - most folks would consider it a fairly standard photographic range with equivalence of 26ish to 78ish mm on a Canon body.
The 10-24 is well and truly an 'ultrawide' and upon first using this type of lens, it can be downright shocking how close you stand to things and still get them entirely in the frame - and how often people who think they're out of your shot are standing firmly in it. I personally love ultrawide, and I have had a great 2 year experience with the Tamron 10-24mm lens...my gallery of this lens is here:
http://www.pbase.com/zackiedawg/tamron_1024mm_f3545_lens
It mostly will depend on what types of photography you want to shoot, and are most likely to need...the 17-50 will be a main walkaround wide to standard lens, while the 10-24 will be an extreme wide specialty lens with some normal wide capability.