This question was borne out of a discussion with a friend. We were talking about meeting future energy demands and we both think a predominantly nuclear approach supplemented by renewables and energy saver (i.e. cavity insulation for all houses) technologies would be desirable. When the conversation came to implementing fusion reactors if they prove viable we stumbled upon the fact that speculative generation IV nuclear fission reactors have about the same estimated time frame for development and implementation and offer great advantages. Which begs the question; presuming that projects like ITER/DEMO succeed in generating commercial fusion will there be that much of a competitive advantage over generation IV reactors? Neither of us are experts and so couldn't judge whether or not our answers would have made sense.