The Shield has a thumb safety, which is not needed but a comfort factor for some. Many people feel that the Shield has a better trigger than the XDs, more Glock-like. Their size is almost identical (the XDs is 0.1" thinner). The biggest advantage of the XDs is that it's .45 ACP, not 9 mm.
I switched to the XDs from a Kahr PM45 because it is more ergonomic (I kept the arched back) and the slide is MUCH easier to operate. I find it more forgiving of ammunition (I use hand loads for practice).
The XDs trigger is okay, but for some reason I get a blister on my trigger finger. I think it's from the safe-trigger extension. The Kahr trigger is perfect for an SD pistol - 4.5# and smooth as silk. The durned recoil spring is so stiff, it is very hard to charge the chamber or clear it. For some benighted reason, Kahr only puts serrations halfway up the slide. The XDs is as easy to rack and clear as a full-sized 1911.
I like the XDs' loaded chamber indicator, and that it omits the unneeded cocked striker indicator. If it's loaded, it's cocked (or a squib), but not vice versa. The Kahr extractor protrudes about 1/2 mm when loaded instead of flush, which is easy to judge by feel. Neither the Kahr nor the XDs has restrike capability. Both have optional 7 round magazines, which are good for backup but too long for carry. If five rounds is good enough for a snubbie .38 revolver, five rounds of .45 ACP should be adequate in a pocket pistol.