Of the two, I would (and did) go with the USP also.
My memory isn't always accurate, but if I remember correctly the M&P isn't fully ambidextrous friendly. The mag release is reversible, but no ambi. The slide stop/release was harder to use. It wasn't capable of all of the different variants that you can get with the USP series. It wasn't as amazingly ergonomic as Smith and Wesson promised it to be. It holds two less rounds than the USP, even while the M&P is a bigger and heavier gun on paper.
And the only two I had seen used in person were choking on two different kinds of ammo. The one I most frequently saw was a guy's who was practically at the range every day I was there. I think he sent his gun back to S&W once for a problem and got it back and it had the same problem. Let it be known that any gun can have a lemon, it's a luck of the draw thing. But when you see two examples fail, it just kicked my mind into "don't buy" mode. Especially since the M&P was still relatively new back then. That didn't keep them from dubbing the M&P "the new standard in reliability" though... I always enjoyed that. Kind of like how Hummer's slogan is still "LIKE NOTHING ELSE..." except the Tahoe... and except the Yukon... and the Trailblazer... and the Escalade...
That's what turned me away from them. Mostly, it just didn't fit what I was looking for in a handgun.