I see the XDS appealing to some because of the following differences.

-Different back straps.
-Ambi mag release that is not as sensitive.
-No thumb safety, but does have a grip safety equally complained about to no end for no particular reason.
-Extended mag carries one extra round over the Shield
-Grip texture is a little more aggressive and slightly thicker for those with larger hands.
-It has a rail for a light which some might like if used for carry and HD.
-Glock like trigger which some prefer since Glock will probably never make a single stack pistol.
-It's available in 45acp.

With all of that being said, you may disagree and counter anything I stated, but it's all personal preference. I owned a 9mm Shield and shot it side by side with my XDS in 45acp on several occasions to see which one I liked better. It took me some time and several range outings to prefer the XDS that just fit me better. I'd buy a 9mm Shield again and XDS if I got either for a deal like my my originals which was $350 for a almost new 9mm Shield and $450 for a almost new bi-tone 45acp XDS.