The 915 was the "value" version of the 5904 and shared the same basically lines. The sights were not Novak and the overall finish was not quite as nice. But there were very good goods and an excellent value.
The 910 took it a step further. They have even less milling on the slide and plastic sights. But again, good guns and a good value.
And 59XX series mags, grips, etc. will work with either model.
S&W Model 910
S&W Model 915