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IMO I think it depends on the spirally effect you're going after. They both can create sprirals on the hair. Some people may have some long hair and maybe opt to use flexi-rods since it's longer than a perm rod and there's not much of any hair overlapping on the flexi-rods causing a nice spirally effect as a result. Now some have also used perm rods and even may overlap as well, and yet still come out very nice. I say it's all in the technique and how you do it really.