While both pistols are very similar, there are a few differences.
The 910 uses some plastic parts. The guide rod, sights, disconnector, and mag catch are plastic.
Another more significant difference, is in the lock up of the two pistols. The 915 has an additional locking lug on the barrel, and a corresponding cut it the slide. In addition to this, and unlike the 910, the 915 also has forward frame rails. These additions improve the lock up and inherent accuracy of the 915.
It also means that the 910 and 915 barrels are NOT interchangeable. Nor will a 915 slide work on a 910 frame or vice versa.
The 910 is no slouch though. I own both pistols. My 910 is an early production blue model, and is approaching 12,000 troubble free rounds. It is accurate enough for IDPA, and reliable enough to be my on duty carry pistol. I only paid $325 for it back in the day, and have more than gotten my money out of it. I did "modify" it by putting in a metal guide rod from a 5906, and putting OEM curved backstrap grips, with a Hogue handall. The only thing I can think of to say negative about the 910, is that the frontstrap is slippery. My mods cured this. It also made the 910 slightly more accurate IMHO.
My 915 is a "one off". It was modified by the S&W performance center for one of the S&W academies instructors. The slide was cut for Novak Low mount NS, and it had a trigger job, and a modified slide stop. It shoots like a laser, and is a great pistol. I'm happy that it is, as I have so much in it, that it will be passed on to my heirs.
Many of the 915 parts, should be interchangeable with the 910. The mag catch, and ejector, as well as the trigger, hammer, hammer strut, grips, and grip pin I know are. All the S&W 5904 parts will interchange with the 915 too. You can actually swap top ends between those two models, and the model 5906 as well.
The 915 was basically a less finished 5904. The 910 is a much less finished 915. Hope this helps. Regards 18DAI.