As others have said, its a personal preference thing. For me, I decided it was time with this round. I had a 5s and, though eligible for the 6, I didn't upgrade then.
Two reasons, I cant deal with the 4" screen anymore and the CPU/GPU in the 6S seems insanely fast which should carry me though for a couple years.
I opted for the 6S instead of the plus as, FOR ME PERSONALLY (no hate plz), I find the PLUS size too large for a phone. My wife has a Plus though, and she LOVES it. Sadly, she likes em big... (sigh)
IN any case, its your choice and I don't think you can make a wrong one. THe longer you wait, the better the phones will be and, in a year, something better will be released anyway. Knowing that this is ALWAYS going to be the case makes it easier to deal with.
Good luck.