It has almost twice the battery capacity as Gear S (570mah vs 300). Hardware wise, both use the same chipset and same amount of RAM and storage.
I don't play music from my watches. So, can't comment on that. But that's probably to be expected especially you have to stream it off the cell connection.
I'm at 81% after 5.5 hours. Have cellular turned off, but wifi and Bluetooth turned on. Funny thing is that two days in a row, watch stays at 100% for 2 hours. Kinda like a gas tank, as the top 1/4 of gas tank seems to last the longest.