iPhones in the 5GHz space

Hi,

I have an 11ac router that offers 11n over 2.4Ghz / 5Ghz and/or 11ac over 5GHz.

I also have 2 iPhone 5Ss and and iPhone 6 in the house.

If I have them all use the 5Ghz WiFi will this mean that everyting, including the iPhone 6, will run at the slower 11n speeds and to get best performance for the iPhone 6 it should be the only device on 5Ghz to ensure it runs at the higher speed 11ac?

Also, as we enter the house will the phones automatically pick up the longer range 2.4GHz 11n signals and not the 5GHz signals. Is there any way they can be made to automatically move to the 5Ghz signal once they pick it up?

Or should I combine the 2 frequencies by having the same SSID for both and let the phones decide which frequency to use all of the time?

Thanks

D.