RSeacord,
The P99 was originally developed by Walther and Walther only. S&W came into the picture after the vice-pres. of S&W was visiting the plant in Germany to discuss the CO2 pistols that Walther's parent company Umarex builds for S&W. When the V.P. first saw the pistol he thought it was ok, but after it was released in the U.S. he made another trip back to Germany and a deal was made. The story was the V.P. of S&W saw a great potential for the pistol to be sold to the law enforcement community and to replace the failing SIGMA line.
I have compaired the two side by side and the SW99 is crap compaired to the P99. I hate the Melonite finish on the SW99. All the brand new SW99's that I've seen have already had some of this finish worn off. And when you pull the slide back it feels and sounds like a bunch of sand is in the gun. The P99 however has a nice Tennifer finish. I know that their QPQ model (which is the silver slide model) goes through a quench of tennifer, buff, and then another quench of Tennifer. Hence the name QPQ! I would almost bet that they do this with the black tennifer also. Walther's have always been known for there beauty and one of the reasons is they take the time to make the gun look nice. Have you ever seen a brand new Walther PP or PPK with its deep shiny blueing? Just Beautiful!!!! The P99 is no different. They use a different finish than the PP series (tennifer vs blueing) but you can tell they still have that pride in making a beautiful, reliable, high quality product.
Go for the P99!!! I did, I've got 4! [:D]