I got a 6 last year. The reason being that from the 5S it was already a nice jump. I also thought that it would be easier to carry, and it would look less silly.
However, a year later, and I think it will make the experience fun for me to move to the 6S Plus, it won't just be an "S" update for me, but a new size too. Plus, it seems to be more common to see gigantic phones now, so you fit in better than you did a year ago with one.
Still, if I find it much too inconvenient, I can always sell it, or perhaps even return it and move back to the 6S.
I'm surprised the Plus models ran out so fast again in the pre-orders. I thought Apple would be better prepared this year for the closer than expected split between 6 and 6 Plus. I wonder if this year that many more people are moving to the Plus than the already surprise number last year.
I understand Plus size isn't very popular in Europe? Not sure if that is true? It might feel a bit odd traveling with the plus, I'd probably prefer a smaller phone for travel, but I don't think you can make that an all out decider.
Unfortunately, the Apple Watch has made it more difficult to go back and forth between phones.