Re: iPhone 6s Plus vs Samsung Galaxy Note 5
Samsung really stepped up their game this round. No lag ever. No crashes ever. Everything is fantastic on the Samsung front. I'm even using TouchWhiz and I freaking hated TouchWhiz before. The issues you'll have are due to Google and Android itself regarding apps not behaving themselves and draining the battery heavily. The App quality is sometimes downright atrocious. You'll be fighting with Facebook a lot, for example. The screen is drop-jaw gorgeous. The S-Pen is fantastic. Samsung Pay allows you to not even need an NFC terminal. The phone construction reminds me of the 4S. The camera is absolutely amazing. Also something you should look out for is a bug on Android 5.0 regarding cell standby drain. That said, I love my Note 5. Don't put your pen in backwards.
My iPhone 6s gets here(hopefully today *fingers crossed*) so I cannot comment on it specifically but I had a 6 Plus. You'll get a quality product with specs that are not as high up as Android devices but what they lack in the hardware department, you'll definitely get in the software. It's a testament to Apple's great software engineering that they made a 8MP camera be head-to-head against 12MP, 16MP, 20MP, ect. for years. App quality is top notch. Some bits like TouchID and Force Touch are the best in the market. While live photos is nothing new, you'll get the best implementation of it I've seen imo. You'll have a great ecosystem with Apple apps that don't decide one day to start running and drain 10% of your battery. You'll have a personal assistant that won't go HAL on you and drain your battery just 'cause. Be aware that the 6s only has a 720p display and the 6 Plus has a 1080p display. They're both gorgeous screens but Apple definitely needs to retire 720p.
I know I'll love both devices but that's just me. I highly recommend you go to a carrier though and test drive both phones. If you get one, you can always return it for the other(at least in the US).
EDIT: If apps behave, usually I can go two days without charging and with 5.5 hrs of SOT but that's my phone with Doze, carrier crap disabled and Google Now cards off. Greenify is also periodically used to hibernate apps.