We have set up literally hundreds of 5th gen Camaro owners with both solid and urethane rear sub frame bushings for street and track use over the last few years and I have never seen or heard of these welds cracking before I read this thread. If this has ever occurred I would be very interested in seeing some documentation. Until then solid has proven to be optimum.
Camaro0220, if you end up making your own be sure to post pictures. That would be cool to see what you come up with.