David,
I'm not sure how to answer your question, except to say go shoot one to compare for yourself.
I can say that I've got one of the first P99 .40's to be imported and have steadily bought a new P99 over the past couple of years and have seen improvements. My latest P99's are more accurate than the older ones. One change that I think effected this was Walther changed the crown of the barrel from rounded to squared.
Compared to your Sig 229, I will venture to say my P99 will be just as good if not better in accuracy. My Uncle has a Sig 229. I have shot it and really liked it. I never put anything on paper, because we were just plinking at can, bottles, etc. But I did just as well with it as I did with my P99. I can consistently do sub 3" groups with the P99 at 25 meters with 5 shots. I have had one occasion where I got a 1" group. But that's not the norm with me.
My first P99 had a problem with the slide locking back. It went away after 100 rounds. However, others who have experienced this problem had to have the slide lock spring replaced. This apparently solved the problem. I've never had any FTF's or failure to eject problems.
So, "How does on find out?" The best answer is to go and shoot it for yourself. If you can't do that, I guess the next best thing is to listen to others and read up on it.
Hope this helps.
P99