Obviously at this point you can't compare their achievements. Novak's 3 Wimbledon titles are an unbelievable accomplishment, and blow away anything Roddick ever did. But I feel like peak Roddick could have also beaten Nadal at Wimbledon or old Fed in 2014/2015. And I can't see Novak in his current form beating Federer at Wimbledon during the prime years.
I'd love to see 2004 or 2009 Roddick against Djokovic in a Wimbledon final. I honestly would like Roddick's chances given his h2h with Djokovic and the matchup issues he seemed to cause Novak. I also really wished to see the same version of Roddick against 2008 or 2010 Nadal. Either matchup could have been epic.