most drag cars go with the chromoly tubing, i dont know if they have to or just want too. chromoly has a higher tensile strength but needs to be welded properly, colded welded or tig. if mig welded there are issues cracking. the weld with chromoly is the weakest point, and chromoly wont bend it breaks. i know that in roadracing a while back there was talk of them banning chromoly cages (scca). probaly because its hard to tell if its welded properly.