urethane or rubber?

[70rs/ss]

I am replacing the body mounts and immediately decided on urethane, that is untill I talked to Wes at Classic Chevy, he pointed out some things I had not thought about. He mentioned that the frames are made to flex and when wheeling they will flex a lot. Urethane does not flex, therefor the body will bare the brunt of the flex which will in time distort the body, break spot welds etc, etc. My question is, how many of you have swapped, what did you use and do you like them, and finally how is your Blazer used (wheeled, pavement, both) Thanks in advance for sharing your thoughts!!