Steve,

I think one thing that has been forgotten in all the responses to your inquiry is expandability. Although you are certainly limited by the capabilities of your SU-800 (or any other CLS/AWL optical control) in terms of useful situations and distance from trigger to remote, you must also factor in the expandability of the system. With an SU-800 and a couple of CLS/AWL remotes like your SB600s, you need only buy another remote like the SB600 to immediately add another remote-capable speedlight to your kit...a cash outlay that only involves the price of the next flash. If you limit your remote control capabilities to only the PW system, then you must buy a new FlexTT1 for every new flash that you add to the system in the future.

If I were in your shoes, I'd probably sell the SU-800 and replace it with a used SB-800 and keep the PWs. With that, you'd have a flash that can:

1. Operate as an on-camera CLS/AWL commander of your two SB600's with all the functionality of your current SU800
   
2. Operate as a third remote (in one of two remote groups) commanded by your camera's pop-up flash.
   
3. Operate as a third remote in your system if you choose to get another FlexTT1
   
4. Operate as a third remote in SU-4 mode while the SB-600s are commanded by the PW system (the SB800 would then be triggered by the flash from one of the SB600s...you'd have to keep your PW system in manual mode for this to work so that any preflashes didn't trigger it prematurely).
   

Visit my Nikonians gallery.