OP, the final release is very likely to be the same as dev preview, they will fix a bunch of bugs and improve stability, but they won't be releasing new features, the only thing I see maybe Cortana going out of beta and hitting a global release, but I doubt that will be soon.
I'm sticking with WP8.1 for the time being. I liked iOS8, but you gotta keep the hardware in mind as well.
So to sum it up, I think you are expecting way too much from a final WP8.1 release if you think there will be new features and you are also packing a little too early as we don't know what the new iphones will look like. Personally if a larger phone isn't released by Apple I wouldn't even consider switching regardless of the OS updates.