I would say you made out like a bandit!
That's LOT of 9mm ammunition, which has gotten both expensive and hard to find these days.
The 915 was the 'less expensive' cousin to 5904. It has a carbon steel slide, and an aluminum alloy frame.
But, aside from the sights, and some other very minor parts, the 915 and the 5904 are virtually identical semiautos.
It's like the difference between a Chevy and a Buick: the Buick might look a little nicer, but the two cars will drive the same, and get the same mileage, etc.
The 915 is a very well made semiauto that has all of the fine engineering of all the other 3rd generation S&W semiautos.
I would advise that you hang onto it, and enjoy shooting it!
S&W is making very few metal framed semiautos, and they will likely be discontinued soon. But, spare parts, magazines, springs, holsters etc. are all readily available, and will be for many years to come.
If you didn't get extra magazines for your 915, I would recommend picking up a few, against the day another 10 round magazine limit goes into effect. You can find them at gunshows, or on-line.
If you go here: Smith & Wesson Mags
You can buy the excellent 17 round Mec-Gar magazines for your 915 for $24. The fit flush with the frame, and are well known here on the S&W Forum to work as well as the 15 round factory magazines.
You can still buy new factory magazines, mainly stainless steel ones made for the 5906/5904, but used ones are very common, due to police trade ins.
Many used magazines will function just fine. Some may need a new spring, which are not expensive.
Have fun with your 915. I would buy one in a second, without all the ammunition, at the price you paid!