This subject often evokes strong feelings by Walther aficionados and owners.
I've acquired some passing familiarity with the 99 series over the course of taking 3 armorer classes for the 99 series, varrying an issued SW99 40, owning a couple of SW99's and helping maintain upwards of 50-odd pistols (including a P99).
Walther made virtually everything for the SW99/99QA/990/990L models except for the slides and barrels. The SW99 series guns used a longer slide stop lever spring, requested by S&W, but I don't specifically know who made that spring for them.
The slides and barrels used in the licensed S&W models are through hardened stainless steel which receive what I've been told is a vendor supplied version of the nitrocarburizing hardening process known as Melonite QP. I was told the Walther slides and barrels are carbon steel which have received a traditional zone tempering process (I forget the spots indicated by this time) and then which receive the nitrocarburizing hardening process under the name of Tenifer QPQ. The difference in the processes take into consideration characteristics of the processed appropriate for the different steel alloys used by the different companies.
S&W requested a number of minor changes in the frames made for their licensed models (rails, some subtle ergonomic features, etc), as well as a change of some sort in the mag followers and mag springs.
S&W engineers came up with some revisions to the slides and barrels in their licensed models for reasons of their own. Some of these revisions included machining the barrels to allow easier clearance between the bottom of the barrel and the top of recoil spring assembly during unlocking/locking and some other minor refinements in the profile/dimensions of the barrels.
Walther continued to make some revisions and refinements to their own model line, as well.
While the Walther loyalists like to point out the superiority of the Walther models (and then argue among themselves about the changes which have occurred in the Walther line which some enthusiasts apparently dislike
) the P99/SW99 guns were pretty much the same in most practical respects.
I've certainly had to replace my fair share of Walther-made/provided parts in SW99's and an occasional P99.
I'll say a couple of things about Walther parts, they're expensive and sometimes you have to wait for them to arrive from Germany ...
Something interesting about the difference between the Walther and S&W models was that while Walther only offered a 1 year warranty on their guns, S&W offered their lifetime warranty on their models, which even included the Walther made parts. This can be useful when something like an ejector has to be replaced, since it's molded into the sear housing block and the block costs something like close to $100 last time I looked. Fortunately, they don't tend to break very often, as I think I've only had to replace something like 3-4 of them over the last several years, and 2 of those guns had received a lot of usage. One had been used to fire more than 50K rounds and the other one was used as a loaner in a firearms class and had been fired many thousands of rounds according to the owner who brought it to me for repair. (All of the sear housing blocks were replaced by S&W without question under warranty, BTW.)
Personally, I happen to think the 99 series is a good one and incorporates some nice features and even refinements when it comes to plastic-framed pistols. I think it wasn't pushed enough by Walther in this country and never really received the recognition it deserved. Once S&W decided to create their own new plastic pistol (M&P), emphasis on the SW99/990L lessened they were eventually dropped from the commercial catalog.
I've been told that Walther has been busy outside the US when it comes to military/LE production.
Another company has started marketing what seems to be a 99 series pistol, too. I don't know what agreement and connection they may have with Walther, although their trademarked logo can be found on a Walther website.
FWIW, speaking strictly from a personal perspective, I happen to think the 99 series is at its 'best' when chambered in 9mm. (Which is also how I happen to feel about Glocks.) I can say that having carried and owned models chambered in 9mm and .40 S&W, and after using a SW99 45 for T&E for a while.
My SW99 9c is my personal favorite of the 99 series, and I have no doubt I'd like a Walther P99 AS 9c just as much.
The Walther traditional double action (AS) sear module incorporates what they like to call an Anti-Stress mode to the single action trigger. You can read about in the catalog/owner safety manual on pages 19 & 20 of the Walther America website manual:
http://www.smith-wesson.com/wcsstore.../other/P99.pdf
I leave my SW99's in the decocked mode where the initial shot is fired from DA, myself, but then I've long since accommodated myself to the DA/SA trigger in 'working guns'.
I like my SW99's and have no plans to get rid of them, which is pretty much how I feel about a couple of Glocks I own.
However, I have no plans to buy any more Glocks or 99 pistols. The pair of M&P's I own and use have pretty much convinced me that any additional plastic pistols I buy will more than likely be more M&P's. (I'll probably only keep my Glock and M&P armorer certifications current, too, allowing the SW99 certifications to expire. I can probably keep my own up and running for my foreseeable needs.
)
Hey, just my thoughts. I'm nothing even remotely approaching an expert on the guns.