Frankly, going from a 6 to a 6+ just gets you a larger device, not more of an upgrade. The reason to get the 6+ is the greater battery life. A lot of the apps on the 6+ just seem stretched, not better. It could be that developers will use the extra screen real estate differently after some time, but right now mostly what you see is just larger stuff on the screen.
Personally, if I had a 5S I would not upgrade. I'd wait until the 6S or 7. There you will notice more of a impact for your money. I'm coming from a 4S, so getting the 6+ is a big noticeable upgrade to me. And all of the features of iOS8 are on board, plus the buttery smooth TouchID, and NFC-based payments with Apple Pay. If you upgrade every year, then you are not going to see a whole out of stuff that makes it feel worth the upgrade. And the size of the 6+ is not optimal if you use the device as a phone most of the time. I consider my 6+ as a go-everywhere-with-me computer 1st, and a phone second. For me, the bigger screen gives me more options to read stuff....I still find I will use my mini for giving takes and stuff, but now I can use my phone more for reviewing notes, reading manuals, PDF, etc. I don't need to always grab the iPad for that stuff. The 4S was totally useless for reading, IMO.
In the end, only you can decide what feels like a good upgrade for you. But other than battery life and a larger screen, you aren't gaining much by moving from a 6 to a 6+.
