I was undecided. Played with both in the stores for weeks. Was still undecided, but ended up settling on the Plus. No regrets whatsoever.
Yes the bigger form factor does take a few days to get used to but once you do get used to it, you will feel like you are carrying around a mini computer. This is not to say that you are carrying around a mini computer, but you will feel like you are. On so many levels the Plus is the superior phone, not my opinion, just the way Apple intended it. As one example, I downloaded the Microsoft excel spreadsheet app (free version) and it was not bad, not as good as working it on an ipad obviously, but for the 6 Plus it is manageable. I could not imagine working that app on a screen smaller than 5.5 inches.
I won't even mention the obvious two advantages to the 6 Plus and the numerous benefits that these bring over the 6, but its the little things like being able to view your home screen in landscape mode and have your email and SMS apps with the two panes.
If you're intending to keep your next idevice for at least two years, also have one eye on the future. If momentum continues about Apple introducing split screens to the iPhone, you don't have to be Einstein to know which phone that will be.