The 915, IMO, is to be preferred.
It is a 5904 with older 669 series rear sight and the frame lacking the dust shield fluting of the 5900 series.
The 910 has a plastic recoil guide rod, plastic sights (different pattern), more angular looking slide and frame. Lastly, the barrel slide lock up is different than the 5900 series.
Both had the slide mounted safety/decocker on the left side only (for right handed shooters).
Also the 915 came with one 15 round magazine and the 910 came with 10 round magazines (with a coupon to allow you to buy one 15 round magazine, factory direct).