There is an advantage to the 18mm shock shaft, but it's mostly in strength, and using one requires a small amount of modification. Just a little. If it was me, I would (and have) stay with the 16mm shaft, just for the simplicity.
The Phase 4 conversion for the WR fork is really quite phenomenal considering what you start with, and it really does end up being a great front fork. However, IMO, the SSS already is a great fork, and only gets better when tuned. You can get about the same results from either, and some guys feel like the open bath design makes a better off-road fork, but it really is hard to beat an SSS fork. You could do either, but I think that I would stick with the SSS myself and work with that.