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Polyurethane engine mounts will not rott, but they don't absorb vibration as well.
Although I might be comparing apples to oranges (as I often do) a lot of guys put poly motor mounts on their 4.0L Jeep's and are unhappy with the vibration and go back to rubber style mounts.
Of course, I bet its safe to say that LS2 and LS3 engines are much better balanced than Jeep engines.
My builder and I looked at them and decided not to. Great addition if you are a regular racer..... I did do the poly bushings on my suspension. They work great....