I am sure this will not be the answer you want to read about, but I thought I would toss in my upgrade from my HTC One M8. Before I get to the phone, I want to explain what caused me to switch from my HTC One M8.
The HTC One M8 was hands-down one of the coolest phones I have ever owned. I absolutely loved it until the Android Lollipop update. I have the Verizon variant, which is actually sitting next to me (I toy with it every now and again). The Lollipop update went fine. I did a fresh install to make sure that I wasn't having performance issues. If anyone else has the Verizon variant, the last update was to Lollipop 5.01 which has a ton of performance issues because 5.01 has a ton of bugs (I am using the present tense since my phone is still on 5.01). The fun part? Google corrected almost all of those bugs with Lollipop 5.1 and it's my understanding that Lollipop 5.1 was amazing. 5.1 was released within a week or two of me getting 5.01 on my Verizon HTC One M8. Yet, here I am in 2016 and it appears my M8 will pretty much be permanently stuck on Lollipop 5.01 because Verizon will probably never push out the Marshmallow update (as of this morning, my M8 says there is no update available).
When I have to boot up Slacker half a dozen times and perform a full reboot on the phone just to make Slacker Radio work without receiving a blank screen, something is wrong. I could recreate the problem every single day and it drove me nuts. Now, I know everyone will just tell me to get a different music program, such as Pandora. However, that is compromising. I should not have to compromise when using my phone. The software should work with apps that are already designed to be compatible with it. I shouldn't have to put effort into making basic things work on the phone.
The next issue is that my phone would heat up when using basic apps like YouTube for 5 minutes or so. The heat was nothing that would be deemed uncomfortable, but it was extremely noticeable. Again, I know for a lot of people this is not an issue because they are used to having phones heat up drastically when playing short YouTube videos. However, I realized that there are other phones out there that do not heat up at all when playing YouTube videos. This made me realize that there is a quality difference between other phones and the HTC One M8 was subpar in this regard.
If I was in an office text stream when I may be sending 50 messages or so in a matter of 10 minutes without turning my screen off, my M8 would heat up significantly as if the screen was causing the phone to get hot. Again, many phones do not have this issue.
The standby battery life on the M8 was also pretty poor. If I left my phone on my desk for a couple of hours without touching it and there was almost no activity with regard to emails and messages coming into the phone, I would still lose a significant amount of battery and the phone was not even being used. I know how to turn off background tasks, disable location services, etc. Anyone who thinks you should have to do this to conserve battery is compromising the experience of their phone to achieve better battery life. There are phones out there where you do not have to compromise to get great battery life, even when the phone is in standby.
The last iPhone I owned was the iPhone 4, which was a great phone and always did a good job, but I was tired of the small 3.5" screen and left Apple to go for Android for a bigger screen experience. I love Android. I did a short stint with the Galaxy S3 (garbage phone), the HTC Droid DNA (great phone, terrible battery), Galaxy S5 (garbage phone), and then the HTC One M8 (great phone, but for issues stated above).
I purchased an iPhone 6S Plus as my M8 replacement. I can tell you that the battery life is absolutely amazing. I don't have to compromise or turn off background activities. If I leave my phone on my desk for a couple of hours with messages coming in (which turn the screen on), I may lose 1-2% of my battery life. I have Exchange Server email, so my iPhone is always pushing email. I can type out 50 text messages without turning the screen off and the phone does not heat up. I can watch an entire CNN political debate, such as the debate this week, for several hours and the phone doesn't even get remotely warm.
Here's the big thing-- the apps all work and they work very well with the phone. I don't have to compromise due to poor software updates from HTC, Verizon and Google.
Software updates? I get them immediately. I do not have to worry about waiting for Google to issue an update and then send it to HTC to mess with who then sends it to Verizon for them to choose to load their bloatware garbage into it or just forget about their customers and not send an update at all. Lollipop 5.1 was released in February, 2015. It's been more than a year and my phone has not seen an update. Lollipop 5.01 is a terrible build. They could have at least moved us up to 5.1.
My iPhone 6S Plus will get the latest updates easily for the next 3 years, which is great.
If you want a no-compromise phone experience as far as being able to use the phone with all of its intended features actively running and still have a great experience, I recommend the iPhone 6S Plus 64GB or higher variant.
If you have to sit there and cripple your phone's experience to achieve good battery life and performance, then there is something wrong with the way the software is interacting with the hardware, or there is something wrong with the hardware, such as the poor Snapdragon 810 processors from last year.
If you have never tried a 6S Plus, I recommend you give it a shot. You may be surprised.
I know there are a lot of people who may think this is blasphemous and start arguing about no "real" multitasking, etc. in iOS versus Android; I encourage those people to at least try the other side. For most things we do on our phones, the two operating systems are remarkably similar and come down to subjective differences.
I love both Android and iOS. But right now, when it comes to a no-compromise experience, I have experienced for better success with iOS on the 6S Plus.
Good luck with your decision.