I suspect that final ratio will be around 60% for iPhone 6 and 40% for iPhone 6 Plus.
Before anyone goes on a rampage attack against why one would get one over the other, I think it's clear that if you have the means to carry it, 6 Plus isn't that unwieldy, and having a big screen and longer battery can be very nice.
Imagine if Apple made only 13-inch and 17-inch MacBook Pro. 17" would've been a lot more popular (instead of commanding less than 10% of the overall MacBook Pro marketshare before being discontinued altogether).
My belief is initially we will see a larger demand for the iPhone 6 Plus until the smaller percentage of ones who want one get one in their hands. Then a slight reduction in demand as time goes on. Even before, a lot of people based on surveys, the largest percentage of people bought 5" screen or under. I think we will see this too.
It very doubtful the 17" would be more popular then the 13". The 13" MacBook still outsells the 15".