I don't like flip covers as an idea. But I'm 100% with you on purchasing the power case.
As I own the S6 I made a bit of a research on the subject. My choice is an extended modular power case. It can give you not just extra battery life, but memory, camera, VR options etc.
I like the Otterbox as a brand by far. But in the sphere of modular cases, I discovered the Nexpaq power case that looks amazing, but it gives only 700mAh. And I also consider I-blades. The nexpaq looks cooler and it has more extensions than the I-blades. But the I-blades case adds more power and a memory card at the same time. I will see how many modules I will need and decide from these 2 choices.