Having owned both the XDS9 and a Shield (which I still have), I much prefer the Shield. As pointed out, it's lighter, less bulky, and cheaper, and I personally shot that thing like ****. The pistol was 100% reliable, but I had a really tough time shooting it accurately (offhand).
I generally like to pocket carry, and the XDS was square and clunky, and just wasn't a smooth pistol, externally (compared to the Shield).
Also, I believe the mags are easier to find deals on, and the Shields seem to have better resale-trade potential ( I always like to factor both of those in, when possible). From what I see on the classifed sites, Shields are very in demand, and get more attention and move faster than Springfields.

edit: I'm hearing great reports on the Shield 45, and I surely wouldn't kick one out of bed. But for me personally, I don't want a 45 pistol with anything less than a 4" barrel. I think a quality 9mm SD round has better performance out of a real short barrel (both ballistically and in cycling).