My Pake-based Fixie is steel and by no means light at 4.95lbs. without the fork, but it is very stiff yet with a lively feel inherant to steel. Aluminum tends to "deaden" the feedback through the frame so the ride is different. For me stiffness is more important on a fixie than weight, and responsiveness is more a matter of where you are going to ride it and the handling qualities that you prefer.