I say everybody wants the 6 plus because everybody would want the new camera features and battery life. The only thing holding back people who bought the 6 is the size of the plus.
OIS on top of EIS will be helpful in only the most minimal of situations. I don't consider this a big plus for the Plus over the 6.
Battery life may or may not be significant between the two phones considering the Plus has to power a million addition pixels. Gotta wait for the phones to see how the battery life fairs in real life.
In my case, neither one of these were factors in my decision to get the 6. I owned a Lumia 1520, so I already know the 6+ is a PIA to use one handed and carry around, much less use in a car mount.