Sorry to hear the S6 has been less than stellar but based on what we've learned about your preference, can't say I'm surprised. Samsung's Achilles heal has always been their software, whether its lack of optimization, overwhelming size and scope, or design and layout. Unfortunately this had meant a brute force technique with their hardware, necessitating tons of raw horsepower in their CPU and RAM and a substantial sized battery. I think in this case they have not yet improved Touchwiz sufficiently and also fallen pray to the perceived need to 'keep up with Apple,' forsaking more battery for a thinner design. And using a QHD display can't help. Yes it may be the most efficient QHD smartphone display yet but no matter how you slice it, 1080 HD would use less resources.
Its a real shame too, because there's no denying these latest Galaxy variants are stunning pieces of hardware, Samsung's best yet, IMO. The camera appears to be the best ever available on a flagship smartphone, the design and build materials are exceptional, and though I find it overkill, the display is beautiful. But as usual, any weaknesses or flaws are attributed its software. I'm not saying is a bad phone--it's still obviously quite good in areas, but think how good it could be if Samsung could actually get their crap together and develop some truly optimized and efficient software. It's exactly why some of us would love a GPE version of this phone, a version not hampered by Samsung's deficiencies.
Regarding Sony, my only experience with their smart phones was with a Z1 Compact for a few months. The hardware was nice--solid build, good camera, great battery performance. The display however was just OK and I didn't like Sony's skin. A GPE version would have been nice but I wasn't blown away.
Can't believe I'm going to say this but you know who should make the next Nexus? Samsung.