I believe with Reachability, iPhone 6 is more reachable than iPhone 5/5S for any corner.
The convenience of reachability is negated by having to reach down to the bottom of the phone, double tapping, then reaching back up. It makes a one-tap step into 3 taps. It's especially if you have to use it multiple times in a row, tapping the home button 6-8 times.