I have both because in Nov 2005, I was looking for a P99 A/S new in my area. I had no luck. I looked everywhere (I won't buy a gun w/o seeing it first).
Anyway - I went to the large Houston Reliant gun show, and had no luck finding an A/S in anything but a 40 cal. But, I did see a SW99 for $437 new. At this point in time, the SW99s were just being discontinued, and they were no longer importing the P99 A/S (only the QA). So, it was the SW99 or nothing. I had been looking and calling all over Texas prior to this gunshow.
I looked at the SW99 a lot, and compared it to the 40 cal P99 next to it. For the price, I jumped at it. It is a great gun.
I shot the SW99 more accurately than a 1911 I had bought right afterwards, too.
The next month, my shop was able to track down a compact P99 A/S, but no full size. So, for a full size gun, it as me and my SW99 until the summer of 2006. It was then, after Walther USA had started re-importing the P99s with the A/S trigger that I was finally able to get my hands on one.
Now, owning both - as well as that SW99 served me for all those months, I do slightly prefer the P99. I find it a tiny bit more accurate. I also prefer the way the P99 looks.
I do notice that 1 handed, the SW99 grip is more comfortable. 2 handed, I like the P99 grip better. Both are more ergonomic than any other gun I have held, though.
And, they are pretty much the same gun. The SW99 frame is made by Walther. The 2 guns operate exactly the same. Neither has caused me a lick of trouble.
I have all 3 hard chromed