applied_force applied over time(over frame)
impulse applied once set
both affect physics attribute of rigidbody
applied force is like gravity
impulse is like force that you apply in jump, or shoot, or bounce
for A, you insist apply impulse over time, in your case(wall sticking) the calculation error /or performance over time is not much, so you get what you get. in other case (which is more complex). this may cause error calculation or performance issue .