As you can see, these two small frame pistols are very close in size. The difference you feel is in the grip circumference, the Sig filling the hand a bit more.
I like the Sig sights better. They are easier for me to pick out. The Sig may point a bit better too.
Both are very accurate so that if you do your part, the bullet will hit what you shoot.
If you intend to carry, then the caliber difference may matter. Most Sigs are .380, most PPs are .32. The PP gives you an extra round. The PP is a little heavier than the Sig. The PP may win in the slimness comparison, especially the grip width.
Both are well made, exceptional quality. Mine have been very reliable. I can't recall any of them failing.
The takedown design is a little different, but both are very simple to maintain and clean.
If asked, I would say I prefer to shoot the Sig. The reason may be the marvelous grip on the Sig, which fits my hand perfectly.
Some people complain of slide bite from the Walthers, however neither of these, the Walther or Sig, have cut me personally.
Pictured are a Walther PP in .32, a police turn-in (note the lanyard loop), and A Sig P230 also in .32, a more rare model than the usual .380 Sig.
Here's the flip side of the Sig, showing the similarity with the PP in that it also has a safety and lanyard loop.