I like the sound quality better on my 6s to be honest. It's louder. Even tried the VoLTE settings on each and ultimately I prefer not using data for calling but the 6s still sounds better to me.
However, speakerphone is a whole different deal. The 7+ I have is very loud and people can hear me really well.
The volume is fine on the 6s. I don't know about the 7 but the 7+ is absurdly loud. Haha.