My friend has a XDS and I have a Shield. We put them right next to each other and they are virtually identical. The XDS is just about as close to a Glock as you can get. But, I like my Shield w/ the Apex parts better. The Shield has a lot more sight and other options as well.
That being said, the XDS 9 will be coming home w/ me very likely. Based on their specs, it will have one more round in the extended mag than the Shield. I like the grip safety more than the small safety. However, I haven't got a chance to shoot the XDS yet, so I don't know if it will have any engagement problems. I had a full size XDM and the grip safety had zero issues.