The iPhone 6 and definitely 6+ is flawed with the limited 1GB of RAM. It's pretty terrible. Apps that were recently open are closed because Safari causes such tremendous memory pressure. Heck, if you have a podcast app playing in the background at the same time as browsing a page in Safari (or any app with a web view), it will close the podcast app!
Sigh.
Most people don't realize that some iOS devices are better than others due to their relative power/performance/RAM which makes them a real sweet spot. iPad 2 and iPhone 5s are the most recent ones I'd say. If anything, I'd recommend buying those models on the cheap and using your upgrade next year on a 6s. Apple Pay in the iPhone 6 isn't widespread enough to bother using your upgrade on right now, and the NFC chip isn't usable by developers so I wouldn't say you are missing much today getting a 5s.