True rubber flexes more than poly, but "new" rubber mounts are pretty stiff (they just lose their stiffness over time as they break-down). Poly stuff doesn't lose as much stiffness over time so it stays harder. Granted the frame does flex quite a bit on GM's (which is great for crawling). I have seen people put solid (as in steel with NO rubber mounts) on and not do any damage to the body (as long as the mounts in the body were in good shape to begin with). I have seen others who put body lifts on with new rubber mounts and with the added leverage on the rubber mounts have had problems (such as the bed hitting the cab on pickups). I'm not going to make a blanket statement and say everyone in every application should use poly instead of rubber, but in most cases it is the better way to go (lasts longer, resists gas/oil/grease/salt/etc, and won't sag).