Handle, examine and try both on a rental range if at all possible.
I own a couple of the SW99's, both in traditional double action, one in .40 S&W and one in 9mm. I carried a SW99 standard model chambered in .40 S&W as an issued weapon for a few years.
I've been to some SW99/P99 armorer classes and the M&P Pistol armorer class. I've repaired both the SW99 and the P99 as an armorer. Haven't had to repair a M&P, yet, but I like a lot about it from an armorer's perspective. Just an opinion.
Both are good designs. Quite a number of differences, though.
Personally, I've always like a well made traditional double action trigger - which some folks like to refer to as DA/SA and Walther refers to as Anti-Stress nowadays. The P99 (and SW99 which uses the same Walther sear housing block & striker assembly) arguably has a very good example of one. I actually find it easier to shoot my SW99 compact with greater practical accuracy, over longer ranges, than my favored 3913 or my G26. Never expected that.
On the other hand, it didn't take me long to become comfortable and proficient with the M&P's trigger. It helped that the M&P 45 I own has exhibited very good inherent accuracy, despite the heavy trigger. The excellent ergonomics, accuracy and predictable trigger more than make up for the heaviness of the trigger ... in my opinion, which is subjective.
Parts for the 99 series can sometimes be a bit difficult to obtain. Sometimes they're on back order from Walther. Been three on more than a couple of occasions.
The Walther warranty was just 1-year last time I checked (even though S&W warranties their licensed SW99/990L versions, including the Walther parts, under their excellent lifetime warranty).
The M&P warranty ... including S&W's custom of paying for shipping & insurance both ways should the gun ever require repair ... might be attractive to some customers.
Folks who buy Walther P99's often express an affinity for "owning a Walther". That's fine. Cool. Lots of folks buy and take pride in owning lots of things in much the same manner. People only have to justify their purchases to themselves (and their spouses
), after all.
I won't be getting rid of my 99 series pistols any time soon. Good guns.
On the other hand, I won't be buying any more of them, either. I simply like the M&P more and more. I might buy another M&P sometime. A compact, perhaps, since I favor smaller off-duty weapons ... and anticipate continuing with that trend once I retire.
Both are very good designs.
Kind of like deciding between different motor vehicles, though. Everybody's got their own idea what suits them best. Which is as it should be.
Good luck to you.
Don't have 'the answer' for you, though.