I looked at the 6 specs and decided to upgrade my 32GB 4S to a "like-new" 64GB 5S. The 5S fits horizontally at the bottom of my front pocket and feels extremely light in comparison to the 4S. I don't think the 6 would fit as well, I would have to carry it vertically in my pocket. As far as speed the 5S is up to 4X the speed of the 4S on some benchmarks whereas the 6 is about 25% -50% faster than the 5S. With a $0 cost with 4S trade-in, a good deal, except I needed some extra lightning cables. I was just about out of space so needed the 64GB of space (too many camping photos and videos and music). My 4S battery also needed to be replaced after nearly 3 years.
DW stayed with her 4S, she is a lighter user and the 4S fits a specific purse pocket.
The only 6 feature I miss is digital video stabilization.