I got my M&P when I lived in MA so the trigger was beyond terrible. I now have a full apex kit with the reset assist and its one of my favorite triggers.
I was thinking about putting an Apex in my M&P, but I will have to find someone to install it.
I've only dried fired the M&P and haven't shot the XD much and have never shot the PPX. So take my advice as such. The M&P trigger is horrible, if the rest of the gun wasn't so great, I would never give one a second thought. The XD's trigger isn't the best, but it's not bad either. But if the PPX's trigger is anything like a PPQ's trigger you have your winner. I own a PPQ and the trigger is by far one of the very best I've shot. It's very smooth, breaks very crisp and the reset is very short and crisp. I've read that the trigger on the PPX's are not like the PPQ's though. I know the P99's trigger isn't like the PPQ's either.
With all that said I think I would just get the PPQ. They are not much more than a M&P or XD. They are a little smaller than the PPX's also.