SW99 stainless slide and barrel, P99 carbon steel slide and barrel.
Receiver internals are identical, frames are cosmetically different. Slide internals are identical.
Fit and finish is slightly better on the P99's i've seen.
SW99 is Melonite over stainless steel, P99 is Tenifer over steel. The former does have an edge over the latter since they improve the lubricity, wear and corrosion properties of the base material.
Tenifer/Melonite are trade names for the same process (Tufftride is another), all owned by the Dufferit division of the HEF group (not Glock). That's the German division of a French mulit- national corporation.
Considering both are made on CNC (computer numerically controlled) machines to the same specs, doubt it matters as much as some think where the machines are or who pushes the buttons on them... unless you are a union man/woman!?
I had previously read only anecdotal evidence regarding the P99 being more accurate than the SW99. However in the April/May 2003 issue of "Handguns" magazine, Phil W. Johnston submitted an article testing Triton .40 S&W 135 gr.
The two guns he used were the P99 and SW99. His range data showed the Smith gun averaged 2.83" groups at 25 yards, while the German gun averaged 1.83". Also the velocity of the Walther was 1,258 fps. vs. 1,213 fps. for the Smith.
Of course this is just one example, but evidence is building that the Walther may be superior.
IMO the P99 is superior to the S&W in all aspects. Fit, finish, accuracy-(I have shot them side by side, a SW99 vs. P99 in 9mm), and trigger.
The SW99 trigger is not a smooth as the P99. It's hitchy, a tad gritty, has a longer pull and a abrupt let off. When I shot my P99 againsts a friend's SW99-he was bragging how his was mine only cheaper-mine was only $20 more than his, I found the P99's groups to be 1/2 to a 3/4 inch smaller than the SW99's, to which I blamed on the trigger. I got used to the smooth trigger on my Walther.
Maybe with a little work the SW99 can be a stable mate in the safe to the P99, but not a equal or a superior.
Fed 135 HS 1200* fps/2.68* v 1192/2.83
Master 150 JHP 1220*/3.38* v 1201/3.50
Rem 165 GS 1173*/2.75 v 1160/2.68*
Black Hills 180 JHP 1005*/2.88 v 999/2.83*
Blazer 180 PHP 949*/2.88 v 941/2.75*
Win 180 JHP 1001*/3.00 v 994/2.88*
Avg SW99/2.92 v P99/2.91
The P99 was more accurate in 4 of 6 loads by a whopping 0.01 inches.
The SW99 was faster in 5 of 6 loads by a humoungus 10.8 fps.
Yeah, there is a helluva difference!
I have two P99s in 9 minimeter BTW. After the NJSP/SW99 fiasco, I just trust Walther more.