The differences between the 910 and 915, visually, are the less refined finishing and plastic sights on the 910 vice the rounded slide and metal sights on the 915.
Internally, the 915 is superior to the 910 by virtue of its lugged barrel and corresponding cut in the top of the slide for tighter lock up. The 915 has a metal guide rod and disconnector, as well as metal mag catch. no plastic or MIM in the 915.
While the 915 is the superior pistol of the two, the 910 is a fine pistol. I had an early production 910 I used for training new shooters. I only had around $300 in it and got over 18,000 rounds through it before I cracked a frame rail.
I would take a 915 over a 910.....and any 910 over any M&P.
Regards 18DAI.