Honestly, having read the GS6 thread there seems to be major shortcomings with the phone such as stuttering performance (maybe due to Touchwiz) and awful battery life. Its specs and build are probably the best Samsung has done so far, but I'm hearing a lot of people switching GS6 to iPhone. The Z5, I'm not sure about but if you like the Omnibalance design go for it. The Z2/Z3 battery life was quite beastly from reading reviews. Z5 looks to continue that.
Back in 2012, I was actually looking forward to the Xperia Z line. I was on my iPhone 4 that was on its last leg. I was following rumors daily about the Z flagship, even starting a thread here on OT. Eventually in Fall 2013, I ended up with the Nexus 5 which I was happy with in the beginning. But the experience quickly deteriorated and battery life went from decent to abysmal. Random apps crashing, phone shutting down, keyboard disappearing when trying to type (this was the most annoying thing), and phone heating up from app background activity. And this was on stock Android with little to no customization.
Two years with Android was enough from me to swear ever returning. Obviously, people have different experiences with Android / iPhone but for me unless Apple majorly changes the quality and direction of iPhone I'll probably stick with them. Fast forward to now, and I'm so happy with my iPhone 6S Plus TSMC Edition®. Last night I thought I plugged in my phone to charge, only to see in the morning that the charger wasn't plugged in. The battery was at 20% after 9 hrs of usage (Youtube, Skype, Music, Facebook, and browsing mostly) and 36 hrs of standby. And that's without using Low Power mode.