In 45 ACP I'd say get a XD or HK USP. In other calibers such as 40 S&W or 9mm I'd say go with the Sig.
The XD has an advantage in capacity, though the magazines are quite large to carry with and reflective stainless steel. The magazines tends to attract more attention during open carry or if your concealment fails, I'm considering having them finished black.
I own both Sigs and XDs and in my experience comparing the two is kind of like comparing a Camry to a BMW, both are quality and will get the job done quite well but not the same thing. I personally compare the M&P, XD and Glock, the CPO program has brought Sig into the same price range which is awesome.
In 3000 rounds fired I haven't had a single malfunction with my Sig P229, in 1000 fired through a 45 ACP XD I've had three failure to fires (all Winchester primers) that have fired with a second strike, problem being of course it is a striker design without second strike capability.
I like the fact Sigs have a more comfortable grip, especially with Hogue side panels, second strike, more established reliablity, aren't polymer, and have replaceable grips.
I have yet to find something in 40 S&W I like more than the P229.