The Prius definitely has more room, but the fuel economy is very comparable. According to the database on this site, the HCHII has an average of 46.0 mpg while the Prius has 47.3 mpg. I own both an HCHII and a HiHy. Speaking just in terms of FE relative to the EPA ratings, I do much better in the HCHII than the Toyota. Also, most people seem to say that the HCHII is better for highway driving. However, if the HCHII doesn't have enough cargo room for you, none of this really matters.