I have a stainless Beretta, a Sig 229, and a P99 in 9mm. Of the guns you mention, all are high quality guns. All have similar capacity (15 rounds of 9mm in the Beretta and Sig, 16 rounds in the Walther). All are reliable, and finally, all are in the same price range.
The differences are that the Beretta has a safety/decocker and an exposed hammer. The Sig does not have the safety, and the Walther has neither the safety nor the exposed hammer.
The biggest difference, in my opinion, is that the Walther is really a mid-sized gun (similar size to a G19), while the other two are full sized (similar size to a G17). If you're planning on carrying, I'd go with the Walther.