Jeff,
Over the years I have used both.
The Rubber mounts are more durable and are recommended from my point of view.
My latest experience is as follows: When I re-installed my Hall Phoenix Rad I had one bad rubber mount so I replaced that mount with a urethane one. I re-used the good rubbber mount. When I pulled the rad, I noticed that the Urethane mounts were not re-usable but the rubber one was still OK. The upper mounts (which were also urethane) needed to be replaced.
IMHO, go with the rubber !
HTH
B.G.
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