The DC8 has hydraulic assisted flight controls...B707 only the rudder is hydraulic powered...all controls have manual reversion. The 707 ailerons/elevators are assisted by 'balance panels'...ie: servo tabs.
The 707 is more automated, especially the flight engineer's station.
The DC8 has the more rubust airframe.
As both used the same engines (usually) the noise was about the same.
The DC8, due to its design, was easier to stretch.
The 707 being first...sold more. Also the smaller series, the 720.
Anything else?