I'd say the Galaxy S6.
Sure, the battery life is pretty horrid. But compared to the One M9+, it's just a better overall device IMO.
The M9+ has a slightly larger battery and dual speakers working in its favor, but the S6 counters those with a faster processor, better camera, features like wireless charging and runs Android Marshmallow as of this time. The M9+ is apparently still on Lollipop.
Also note that you won't be expanding the amount of internal storage on the S6 as it lacks an SD card slot.