I think the 6 Plus is going to be more popular that many thought. However, I also think early adopters (we losers who are waiting to pounce at 12:01 am) are more likely to be "power users" or tech nerds, and frankly males, who will be more likely to buy the bigger screen than the general public.
I'm going to guess that in the end, once the mass public buys in, the 6+ will be something like 25%-30% of iPhone 6 sales. I'm also going to guess early sales of the gold color will be very low, but grow later, again, as the general public starts buying (but it will still be by far the least popular color).