Blasphemy! Lol, jk... But seriously 3 different configurations give all 3 rifles a different feel. C7 clone = fixed stock, 20" barrel, elcan optic... M4 = adjustable stock, 14.5" barrel, Aimpoint optic... AR PDW = adjustable stock, 7.5" barrel, reflex sight, 300 BLK...
As for the Tavor vs PS90...