The cylinder which is filled will always reach the bottom faster of the hollow cylinder down the ramp because of Newton's 2nd Law. The moment of inertia it takes to get the hollow cylinder going is greater due to the nature of the equation I = (1/2)*M*(R2^2 - R1^2). I - Moment of Inertia M - Mass R2 - outermost radius R1- inner radius In other words, it takes more time and energy to get the cylinder with its weight on the outskirts than it is on the inner. Think about swinging a yo-yo vs siwinging a rod of the same weight of a yo - yo over your head. You can tell that it takes more effort to swing the yo yo