The X7s are stronger and faster than the X5 series. They were designed with hive minds and are capable of communicating through ultrasound waves (Joshua explained "they have bat in their [genetic] cocktail"), but do so without opening their mouths. Their communications are inaudible to non-X7s. They never speak.
A notable difference in appearance is that their eyes are pure black. They are less human psychologically than other transgenics—far more ruthless and obedient. At least some of them are cloned from the X5 series, identical to their originals apart from their age and their X7-specific traits. Clones of Max Guevara and Zack were witnessed during the '09 escapees' attack on the Manticore DNA database. X7s have been known to be cloned many times over, whereas there appears to be two at most of each X5 template.