OK heres my take on it and its somewhat biased. I was kind of just in the same boat you are in. The two pistols I was looking at next were the XD45 and the MP. All my pistols were 9mm and a few .22lr but I wanted a .40SW or .45ACP. I chose the XD45 and not becuase it was "bigger" than the MP .40SW but becuase I already had a damn similar pistol to the MP. My favorite guns and my carry guns are Walther P99 and P99compact all in the AS configuration which are DA/SA in 9mm. I love the guns they have been perfect and the DA/SA trigger os really cool. If you ahvent noticed the new MP has most of the same features the P99 has. So if I were to get a gun like the MP in .40 I would get the proven P99 in .40 . I have no doubt that the MP is or is going to be a great pistol but IMO the P99 is better(hence the reason S&W quit importing the Walther P99's in AS(DA/SA) configuration. Why bring in the direct competition? It would be stupid. BTW S&W imported 90+% of all the P99 AS's.).
On to the XD45- I love this pistol. The accuracy out of the box was suprising to me as this was my first XD. I hadnt even shot any XD before this. I absolutely am in love with this trigger. It fits the application well. It eats anything. Ive fired close to 1000 rounds of Wolf through it and havent had the slightest problem. At this time it is my nightstand gun. Attached to it is a Streamlight M6 with Corbon +P's . When the winter comes back around Ill carry it some.