Well I own a Sigma .40 and it takes a little getting use to. After about 3 months of shooting the trigger came down to about a 7lb pull.
The XD-.45 with a 5" barrel is a real shooter and accurate. A little large for me.
I never shot a MP so I can't say anything there.

These guns are so similar I think you will need to rent or use them to provide a proper comparison. One difference is barrel length as the full size M&P is 4.25" while you can get an XD in either 4 or 5" barrel. Unless you're carrying concealed, I would prefer the longer barrel but you may not.

I ran across the same choices when I bought my Sigma .40. I was also looking at the XD-9. I did finally get to shoot the XD-9 at a gun range, and its a blast to shoot, the trigger pull is quite a bit lighter than the Sigma. I'm not complaining about the heavy trigger pull on it, but it does take some getting used to. The XD-9 is on my list of guns to get. I haven't got to shoot the M&P yet, but have heard alot of good things about them. My recommendation would be the Springfield XD, unless cost is a factor. The Sigma is a great gun for the money.

I have both (m&p 40 and several XDs) and like em both equally. If I had to choose only one or the other.....I would have an extremely difficult choice but probably would lean towards the XD45 because it is a .45ACP. Fortunately I don't have to choose only one. In fact I will be adding a M&P .40c and M&P 45 to my collection as soon as they are available!

I would have said M&P 100% before Springfield started putting on a better finish on the XDs earlier this year. But, I think I'd still rather the M&P when given a choice between only those 2.

Both make a excelent pistol but I only have experience with the MP's. I've been shooting the full sized MP40 for a while and just recently, bought a MP9 Compact. Very accurate and never a failure of any type yet. I'll probably be buying the MP 45 when it comes out early this year.
My 2nd & 3rd choice would be the Walther P99 & then the XD. That said, for some reason, the Walther is about $100.00 higher than the other two pistols if price is a issue.

I had an XD9 that shot pretty good. But once I got the M&P I knew there is no reason for me to keep the XD. The XD was OK but I like the M&P so much better. Whenever works for you though, XDs are good.

I have an XD40 Tactical and had an XD9 Service.
Extractor problems with the XD9 resulted in a swap for an M&P9

I have put 2000+ rnds thru the XD40 without any problems and realy like the pistol.

I put 600 or so thru the XD9 and have about the same thru the M&P9.

I prefer the M&P9 over the XD9 because it simply feels better in my hand. It also has a significantly lighter recoil spring than the XD9. The recoil spring was a problem for my wife and a couple of friends.

If S&W comes out with a 5" .40 or .45 I will probably give them a try. I like the extra length.

Enjoy