I'd say they're about equal. 2nd form Cell's was meant to make him superior to someone who could stomp him prior, though Perfect Cell's use of his full speed brought him on par with someone who viewed Goku's full power as playing around (albeit, Cell was described as a step ahead of Goku) and who was shown to be vastly stronger than that Goku based on Boo Arc scaling and statements for Gohan. If I were to have to choose one as the marginal winner, I'd say 2nd form Cell's as in the case of true speed Perfect Cell, he still wasn't at the level where he could deal more than superficial damage.
Edit: Thought it was Cell VS Goku powering up to his true speed. Cell's power up to full power should definitely be greater given the context and hype around it.