Urethane would probably weigh a little less, but not much. It depends on the size and function of the bushing. Since some of us may be watching ounces now, then any little bit would help.

RMCHR, I differ on the arms. I weighed my stock UCA's with bushings and all a ways back compared to the Magnum Tubular arms with bushings and all, and the Tubes were lighter.
My stock pair of arms weighed 10-1/2 lbs and the Tubes weighed 8-1/4 as a pair. Saved 2-1/4 Lbs total. This might be that the Tubes have Urethane. My lower factory stock control arms have the stock rubber bushings though.
I just had made a couple of small items that saved a few ounces.
I'm now at 126 ounces into 1600.