Well, in theory, it would be a little smaller - just because you wouldn't need to carry the weight of the combustion engine. I don't think the energy density was much better. But, because you would never have to replace it, you could even build it into the frame. In theory.........

I would love for this to happen - but so many great theoretical technolgies never seem to work (hot fusion, cold fusion) or are just too expensive (fuel cells).