Of course I plan on buying a SW99 in .45 ACP ...
As a SW99/P99 L/E armorer I have a passing familiarity with the pistol design, some of the differences, and the reasoning behind some of the differences. A layman's understanding, anyway.
Aside from the ergonomic/functional/cosmetic differences in the frames, requested by S&W, internally the frames are the same. The same frame parts used in the Walther pistols are shipped along with the Walther frames for use in the SW99 pistols.
The slides & barrels are different in materials and shape for valid reasons.
Cosmetically ... well, that's why they make so many different car designs, isn't it? Also, S&W decided to use some broach cutting equipment developed for the Sigma slides. And before you throw your arms up in the air about the Sigma name, the resulting SW99 slides DO offer some advantage in respect to design and slide mass issues. Chose as you wish ...
I like being able to easily strip the magazine from the grip frame if it becomes stuck, and it's easier to accomplish with the S&W, but only because they specifically requested the front of the grip frame be relieved so the "toe" of the magazine base pad could be accessed more easily. No big deal in ordinary circumstances. I like the thumb recess better in the Walther, but that's simply another personal preference issue.
I like the accessory rail design on the S&W version, again, done that way for a specific reason ... and Walther will offer theirs accordingly soon ...
I dislike square trigger guards, but that's a personal choice that can be defended simply on the basis of whatever the prospective owner desires for themselves ... others like them ...
The little "walking" trigger pin issue, such as it was alleged, was resolved in the Walthers. I know of a fed agency that was briefly T&E'ing some of the early Walthers, and the major complaints by their folks were the small pins working "loose" in the trigger assemblies, and the trigger guard "bump" which irritated their fingers under extended shooting.
I prefer a stainless steel slide & barrel, especially a double heat treated barrel, compared to the carbon steel slide & barrel of the Walther, with its traditionally tempered barrel via heat induction method. Think of it as zone tempering in specific locations of the barrel, instead of the entire barrel. At least, that's how it was explained to us. Nothing wrong with the older method. The German pistols have done well for many years with their methods. No big deal either way. I simply like the double heat treatment of the SW99 barrel.
The enhanced surface hardening is quite similar, allowing for the required difference of the carbon versus stainless steel material issue. The QPQ of the tenifer carbonitriding process used on the Walther is unsuitable for stainless steel, but offers some enhanced corrosion resistance to carbon steel. The gascarburizing process used on the SW99, which is a QP Melonite process, is suitable for stainless steel, which already has enhanced corrosion properties, so the QPQ stage of the Melonite process isn't really necessary, anyway.
S&W engineers have helped contribute to enhancing the 99 series, to the benfefit of both manufacturer's models. Little things, like springs, dressing an angle on the locking blocks, radiusing the extractors, changing a bevel on the bottom of the barrel to enhance the unlocking of the barrel so it better clears the recoil spring during the cycle of operation ... and, of course, discovering how the design of the early 12-round .40 S&W magazines were involved in the reported premature slide-lock occurrences that caused many early Walther P99 .40 S&W owners to become frustrated with their pistols. MecGar used the results of S&W's research to redesign the 12-round .40 S&W magazine bodies and followers, which resolved the problem. Unfortunately, in the US the magazine ban limits non-L/E to 10-round magazines, which fortunately didn't exhibit the same problems as the 12-round magazines during extended testing. I bought several 10-round .40 S&W magazines against the time I hope to retire in another couple of years, in case I'm not given the 12-round magazines by my agency at the time of my retirement.
Both offerings are fine weapons, and are selling as fast as their respective companies can produce them.
Walther has the advantage of offering the P-990 (DAO) and QA variants. They also offer some finish variations.
S&W has been working on developing a .45 ACP version, with the slides & barrels produced here, and using a newly designed Walther frame produced in Germany.
Both are going to offer "compact" versions of the 9mm and .40 S&W calibers. Walther may decide to offer a .45 ACP version of their own, since they're already making the frames for the S&W version under contract for S&W. I'm sure it'll be a fine pistol if they do ...
After the initial minor glitches were resolved in both the Walther and the S&W .40 pistols, they're fine pistols. The 9mm pistols are very nicely made in both models.
I've fired many thousands of rounds through the SW99 pistols, mostly in .40 S&W, but at least a couple or more thousand in the 9mm versions.
I carry one in .40 S&W.
I plan to buy a .45 ACP version.
I'm NOT a S&W or Walther salesman, though, and actually couldn't care less what anyone else does, or doesn't, buy ... or their reasoning ...
Tired subject ... but you DID ask ...