I have several steel/aluminum SIgs and love them all, but for my newest tupperware gun I chose the XD over the Sig pro for three reasons:
1. Standard rail vs proprietary rail
2. Size. Sig does not make a polymer handgun in a compact or sc size. My SA XD9 SC is tiny (6.2").
3. The ability to accept mags from other SA XDs. Sig Pro=sig pro mags only. XD9 SC also take full size XD mags.
they both shoot and point well, and were close to the same price, but the XD had the edge. Also I didn't like the decocker on the Sig Pro - wasn't like the decocker classic series. Just didn't feel right. I might still get one later though, just not as a primary CCW. I alternate the XD9 SC and a Sig P239 .40 for CCW. The XD is quite wide, and less comfortable than the P239. But I like having the double stack capacity, and I carry a 15 rounder as a spare mag.