When you really look at the specs the slight improvements on the + are not enough to really change anything. OIS is nice and there is a very slight battery bump, but other than that it is just a bigger slightly more dense screen.
I feel like it's hard to tell until they're out. It is going to take some time to see what the front-runner is and what the public/company pays the most attention to. I could see it going either way so there's nothing but speculation right now. So to get into a bit of that, I'm guessing the Plus will be the winner of new iPhones.