I'm a long time user of the SC-17 which I used many years ago with a F4s and a macro lens. I recently picked up Nikon 105 f/2.8 D micro which I'll use with my D2x and would like to take my SB-800 off camera. The SC-17 works but I might pick up the SC-29 if there are better benefits with the new cable. Hope this makes sense and I'm looking forward to your answers.
As noted the SC-29 includes an AF Assist Light but no TTL ports. The SC-17 is the older version of the SC-28, and has 2 TTL ports which can be wired to additional SB-800 using SC-18/26 TTL cables. Not much has been written about the old TTL and the dSLRs.