I'm relatively new to the Walther P99/SW99 party, so take my opinion with a grain of salt.
My favorite Walther "guru" is Shipwreck, and he has both. I think he says one is just as good as the other..
The SW99 comes with a lifetime warranty from S&W, which has an excellent customer policy. The Walther has a one year warranty.. From the same folks..
I'm an old fart and a lifetime warranty just isn't a big deal to me.. If I were younger it would have more attractiveness... On the other hand, I don't remember ever having a problem with a new pistol/revolver of any quality, so haven't needed even a one year warranty.
I'm just "gun snobby" enough to enjoy having the Walther logo on my pistols as opposed to Smith & Wesson. Some folks say the Walthers, all German built are better than the Smiths, but I doubt that, and even if they are it can't be by much.
Did I mention I just enjoy having the Walther logo on my pistols?
I'm an old Walther lover anyway. I had a PP in the past. I have a surplus P1, and a surplus P5 now.. So my P99c/AS and my P99/AS fit right in.
My buddy has the SW99 and is quite happy with it.
I know one thing, I love the "AS" trigger/striker very much.. More accuracy than I can take advantage of.
Best Wishes,
J. Pomeroy