imported post
The Sig platform is definitely a finer system, although not necessarily any more effective.
Ignoring metal vs plastic issues, trigger pull is probably the biggest difference.
The XD system has a fairly long, and definitely not crisp out of the box double action striker pull. I totally prefer a Glock pull to it, and I totally prefer a Sig trigger to that of a stock Glock. So I am not a fan of an unmodified XD trigger. Others are. But if you like that system and want a better trigger, plenty of modifications are available, and of course you might not mind it stock.
On the other hand, the Sig has a heavy double action first pull, with subsequent pulls on DA/SA models rivaling the crisp pull of a well made 1911. The transition from DA-SA can be a difficult thing to master, and it can lead to accuracy issues in high stress environments. It also leads a lot of shooters to "cheat" by cocking the hammer manually before shooting at the range, which is a terrible mistake. If you have a Sig or any other DA/SA, I think range practice with a lot of drawing and shooting a double tap DA then SA is a must.
One other difference, which may or may not mean much to you, is that the magazine release on an XD is ambidextrous, whereas I don't believe Sig has ever made a model like that.
Both are as reliable as hammers when clean, lubricated, and fed a steady diet of good ammo with good magazines. It is a matter of personal preference, and perhaps budget. What fits your hand the best may also be a factor.