I was in a similar situation. I'm running out of space on my iPhone, and ditto for an iPad. I had budgeted for a new iPhone, and was strongly considering the 6+. Until I tried it in the store. For me, it's like the reviewer who said both too small and too big.
Another consideration is that Minis are NOT like big iPhones in a couple of ways. Obviously, they aren't phones in that even with a SIM card they are data only. A huge problem with Minis is that if you don't get one with cellular data (like you already have another device with tethering, or just don't need it) then you are SOL for navigation because they don't have GPS. Furthermore, the 6+ cannot use the camera connection kit, so you can't even copy photos from your camera without wifi, which is too slow anyway for bulk transfer. Meh.
My solution was to stick with the iPhone I had. But to offload some of what I used it for, like navigation, to another device. So I got a Samsung Tab Pro 8.4". It was a third of the cost of the Mini retina Haven cited, has GPS, no cellular (don't need it), a better screen, internal micro SD storage, a USB port (I can use SD cards, keyboards, HDMI and even ethernet through it), and is about the same size. Not quite as fast, 16:10 aspect (some like that better, some don't). No NFC and payment. A benefit is Android; there are some OBDII and nav apps that are Android only that I can now use. And it works with BT GPS dongles I had that don't work with iOS.
I find the Mini (or any 7" or 8") format way better than the bigger iPad I had, especially since the screens have gotten much better. Easier to stow, handle and view, at least for me.
So if your need for the 6+ was based in part on the need for a bigger screen, consider the smaller tablets. There are also some Windows tablets in this size for amazing prices. And just use it in addition to your existing kit, rather than trying to get a one size fits all that might turn out to be a one size fits nothing.