AFAIK, both terms are used interchangeably for the same material: "Polyurethane."
Proposed advantages: is supposed to be firmer than rubber, and won't distort as much with age/heavy loading--but not as harsh as solid bushings.
One drawback is said to be that they can be noisy/squeaky if not properly greased when installed.
I replaced all the rubber bushings with 'urethane on my old Falcon Coupe (back in '05 or so), and I think it improved the ride quality a fair bit. I'd not had any issues with sqeaking to date, either.
Hope this helps...
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