$150 is not a huge difference in price. I own both the shield and XDS and I can tell you without bias, that the XDS is a nicer gun. (And I consider myself an M&P fanboy)

The overall construction is nicer and more robust. Obviously to handle the .45 cartridge. It also has a loaded chamber indicator and grip safety. (may not be a plus to you, but it is added cost) Personally, I appreciate the features. Add in the fiber optic front sight, also all the stuff they throw in the case. Granted, I don't use any of it, lol, but for someone just starting out, they could utilize all of it. The magazines are nicer as well.

The slide on the XDS is beefier and has a slightly nicer finish IMO. Also, the grip is very aggressive from the factory where as on the shield you will see many people adding a grip sleeve or stippling to it. The XDS has interchangeable back straps to customize it to your hand size. The Shield is fixed.

The XDS trigger is also better out of the box IMO. I now have an APEX DCAEK ($80) in my shield and the PRP spring kit ($15) in my XDS. The XDS still has a more precise trigger feel with a crazy nice reset.

If I add the price of the APEX trigger, plus the new sights and time put in to stipple the grip, just to bring the shield up to snuff with my XDS, I'd say the investment is pretty equal.

So I can easily account for the price difference in my mind. Sometimes I want to carry light and thin, and I take the shield. Sometimes I want 230grain JHP. Depends how I feel that day.

I will say that the XDS is super accurate. I'm talking Tack Driver! I'm pretty dam accurate with the shield as well, but the XDS is just dead on.

But for the fan boys...I did let Costa shoot my shield lol. He shot it very well.

Costa trying out the APEX trigger on my M&P 9 SHIELD - YouTube