I used to carry a XD40 service and now carry a XD9SC. The .40 is a snappy round, and putting it through the 3" barrel only compounds that, IMO. Plus 9mm is a lot cheaper to shoot than .40.
The SC is easier to conceal, provided you are talking about either the smaller mag or the small mag with a Pearce extension. If you put the extended mag on it, then it's the same grip length, so it's a moot point as far as concealability.