I'm very unsure. Up until the announcement I was leaning towards the 6, but I want to walk into a store and hold both in my hand first. I'm annoyed that they didn't just keep the 6 the same size (thickness) as the 6+ and then done away with the camera lens sticking out. That way they could have perhaps added IS (image stabilisation) to both models and given the smaller model some slightly better battery life. It would have still been thinner than the 5S, but they really are about beauty and not functionality.
Having seen the keynote and looked through the specs,
AND having recently had my 5 replaced due to the battery replacement program, I'm thinking I might wait for the 6S that will hopefully have bigger speed improvements, but retain the same package (though maybe, hopefully, get rid of the camera bulge).
Icing on the cake is that the Apple Watch works with the 5 upwards, so I could buy the watch next year, it'd work fine with my current phone and also next years upgrade.
Of course, all this is meaningless if Telstra offers a nice incentive to upgrade through our business plan. I'm not holding my breath though.
Wonder why they switched from S to plus?
The "S" designator has always been the second release of a model, a speedbump but not a change in design. You had the 3G, and in the same package the 3GS. Then the 4 had a new design and the 4S brought improvements but it looked much the same. Again, they had the 5 and a year later the 5S. The 6 is a new design, the two screen sizes don't warrant an S, next year they will if they keep the same relative design but improve the internals.
The better question would be why they ditched "Air" or "mini" to differentiate models, as they have done for the iPad or iPod. I'm glad they did though, seems less gimmicky than "Air".