I'd go with the Shield, all day long. I had some subcompact XD a while back, but never really dug it, and let it go. I had a Walther PPS (M1) and liked it ok, but when a trade deal on a Shield stumbled my way, I let a freind trade me out of the Walther, and never looked back.
The Walther's problem is it has all the blocky,squared edges just like the XDS has, which serve no function but to give it that nifty-keen,Klingon battle cruiser look that many folks are attracted to (I'm a fan of pocket carry, so the external ergos matter to me).
As mentioned above, the Shield is cheaper, shorter, lighter, externally smoother, and has arguably the best CS/warranty dept, and probably the best trade/re-sale potential.
Also, I got a spare 8-rd mag this past week via ebay for $21.49, delivered (from gunmagwarehouse, IIRC). I don't know about the XDS, but I know you can't touch Walther mags for anywhere near that.