SW99 is no longer made - they stopped in 2005. It was the Smith clone of the P99 w/ the A/S trigger. Walther makes the frame for S&W, and S&W makes their own slide and barrel.
They only make the SW990L now - basically it is the same as the P99 w/ QA trigger, only there is no decock button at all.
The SW99 and P99 are pretty much identical - just aesthetic differences. Slide differently shaped - minor grip changes.
I have both - to me, the SW99 feels more comfortable in a 1 handed grip - the P99 feels more comfortable in a 2 handed grip. I find my P99 to be a tiny bit more accurate. But, it is essentially the same gun. The P99 has tennifer coating; the SW99 uses Melonite (pretty much an almost identical process to tennifer).
I hard chromed both of mine. The Walther looks a bit cooler.
Even though they stopped making the SW99, U can still find new ones floating around, as it has not been out of production long...