Do both. IMO neither is better than the other, and any good jump program will have you doing both. They both work basically the entire leg, but the squats put more emphasis on the lower body anterior chain ( quads,abs), and deadlifts put more emphasis on the lower body posterior chain (hams, glutes, lower back). Some say deadlifts ( specifically stiff-legged) are better because the starting position is more similar to a standing jump position, but I really don't think that means much.