Magazine capacity is going to be listed on Springfield's website, so we shouldn't have to answer that part. A little bit of work on your part would get you that answer. Heck, it's even listed on third party sellers' websites, usually.
As for which one is best for you... No idea. Because handgun choice is an incredibly personal thing.
But you say that you want a target gun, essentially. That rules out the XDS FOR NOW. Not forever, but for now.
Because the XDS is made, almost entirely, as a concealed carry gun. Sure, it's a good shooter, but it isn't the gun I'd take to the range for formal or informal target shooting if I had a comparable full-sized gun.
The XD(M) has a match-grade barrel, so it might be a bit better for target shooting. It'll cost you a little extra, though. Maybe $100 more... But that may be well worth it for you.
You say that SOMEDAY you might carry concealed. Well, I say that SOMEDAY you should buy a gun specifically for concealed carry. I have two handguns. One is a full-sized pistol that I've had for years, the other is my concealed carry gun. Nowadays when I shoot, I do tend to shoot just my concealed carry gun, but that's because my full-sized gun is a lower-quality gun that really doesn't offer anything other than capacity over my carry gun.
But you're talking about comparing, essentially, apples to apples. All of the XD handguns are good. They'll be as reliable and accurate as anything on the market, provided you maintain and practice with them.
I would strongly recommend that you look at other comparable guns, such as the S&W M&P series, the various Glocks, and pretty much anything else an indoor shooting range has available. Although I disagree with "Carl" that how a gun feels in your hand is the first priority, it's certainly important. More important than that is how you shoot the gun. I hate the way a Glock feels in my hand, by and large, but my experience with the Glock 19 is that it's incredibly accurate for me. The only gun I've ever shot that is on par with a Glock 19 FOR ME is my carry gun.
Anyone who tells you a specific gun is right for you is doing you a disservice. You need to make the call, and while other people's advice can certainly help, it shouldn't make the decision for you.