I have an M&P40 and XDm9, both full size.
For me the M&P is more comfortable but the XDm has a better trigger.
If you're gonna carry, the M&P feels a little more compact at least between my two, but it's not much difference.
Both extremely reliable, have over 2K rounds through them with various ammo and not so much as one hickup. I keep em clean though.
You really need to hold/shoot both and decide for yourself, you can't go wrong with either one.