I own both but carry the Sig.
Sig makes three different models of the P232. The Sig is lighter by a good margin in it's alloy frame form. Both the Walther and Sig lock back on the last round but have no slide release lever. Both have a fixed, fairly accurate barrel. Both take down quickly, although I prefer the Sig method personally. The Sig is the better fit for my hand and I shoot it better than the Walther, but that's me.
As to cost; I paid $335 for my Walther and $315 for the Sig I carry, (I have two others that cost a little more). Both were used but in outstanding condition. They are similar enough in size as to be able to share
some holsters. I prefer the regular sights on the Sig and night sights are available on new Sigs (must be factory installed), whereas I don't know if night sights are even available on the Walther.
Both have similar yet different triggers. The Sig is lighter in DA mode at 9.5#, while the Walther feels heavier, at around 14# I'd guess. Both have a light SA trigger at around 4-5#. I never use the safety and prefer not to have one, so here the Sig design suits me better...others may want a safety. Both guns have excellent aftermarket grips available. I use Hogue on the Sig and Pachmayr on the Walther.
Very similar guns and your preference may boil down to which one you like or feels best. Both fine, quality pistols.
Other options for a quality 380 acp are the excellent Beretta 84/85 and the harder to find Sphinx AT380, a very compact, well made, DAO pistol.