Squats = full body exercise
Leg press = leg exercise
When i work out i like to use exercises that i get the "most bang for my buck." The squats work all the major leg muscles plus lower back and abs and stabilizers. I would think only doing leg presses could result in some major muscle inbalances. So If I could only due one i would choose squats because i think it is a more effiecient exercise. I will also add, I hate to squat but i still do them .