The iPhone 6Plus is no fish no meat ... I had one in hand a few days ago and had a similar question in mind: would an iPhone 6Plus replace my 5S and the iPad Mini? My personal answer was simple: No.
I have an iPad Mini Retina mounted to the yoke and the iPhone as backup at the side of the cockpit. The bigger screen of the Mini is almost perfect in size, even though the mount at the moving yoke is not the best solution.
When I looked at the iPhone 6Plus, my first impression was positiv for a possible replacement of my two devices by one. But, after a while I weighted requirements and came to the opposite conclusion.
First, at the iPhone 6Plus you can not disable the mobile network without killing the GPS (same as now with the 5S). Consequence, if you have a problem with electrocoupling in a cockpit you can not switch the mobile net off, second consequence, you could heal it by an external GPS, but then you just replaced two devices by two.
Second, redundancy. If something goes wrong with my iPad, I can switch to the iPhone Backup, or other way around. There are two almost identical devices doing the same. Maybe this is over redundant, as there is a GNS430 also in the plane I usually fly, but I got used to the relaxing knowledge of three independent options (GNS on board power, iPhone and iPad standalone - even Skydemon on both and for additional safety a copy of AirNavigation Pro as well, just in case SD encounters a principal problem). Consequence, I would not replace the two devices by an iPhone 6Plus, but keep the Mini.
Third, carrying. The iPhone 6Plus is big and if you have it not mounted to the plane, but carry it somewhere else, it is too big for convenience. I am not even quite sure I like the iPhone 6 with its increase compared to the 5S. I am not wearing these sloppyjeans with oversized pockets, so the Plus is really something you have to start thinking how to store it. I don not like that Bigness.
So, short. If I am going the route I will just replace 5S by 6 and not go Plus.