Fundamentally, the decision on the TS-E 17mm vs. the TS-E 24mm comes down to this: one is 17mm and the other is 24mm. First and foremost, lens selection is about what you want to shoot, and what focal length will capture those subjects the best. If you need wider that 24mm and can't back up sufficiently, you need the 17mm. If you'll find yourself cropping those 17mm shots in post, get the 24mm. One consideration is that the 24mm is wickedly sharp, and the 17mm, while excellent, is not quite at the same level of sharpness as the 24mm MkII (but the 17mm is still better than the 24mm MkI).
Do you have those focal lengths covered by a current lens or lenses? If so, look over your EXIF or go shooting and see which focal length would be best served by a TS-E. My choice (which I made late last year) was the TS-E 24mm II. I do have an 82mm CPL that comes in handy, and would not be an option with the 17mm lens. Like ronderick, I was a bit worried about the lovely but vulnerable front element of the 17mm TS-E. But lenses and cameras are just tools, so if I needed 17mm, the 'risk' to the front element would have been ignored. 24mm was the right focal length for my needs. But scalesusa's question is relevant - if I was using it on a crop body, I'd have gone with the 17mm.
One more point about the TS-E lenses - it's not listed in the specs, but the Canon extenders work with the TS-E lenses, so you can change the FOV if you have a teleconverter.
Here are a couple of shots from my TS-E 24mm f/3.5L II, taken with a 5DII. The first is the Custom House Tower in Boston, the second is a gate in the Confucius Temple area of Nanjing, China (click for larger versions).
