Jupiter has about twice the density of Saturn (1.33 versus 0.69 g/cm^3) because it apparently has a higher mass percentage of rocky core and of metallic hydrogen in its interior. Available density references of this hydrogen state differ widely from 0.3 - 2.8 g/cm^3. Is there a theoretical density of metallic hydrogen, and would its density remain relatively constant given the varyng high pressures and temperatures found in these planetary interiors?
There will be a density gradient of the hydrogen for the planet, but you can probably quote an average density for the planet if you need to estimate something.
There was work done by Ashcroft in 1968 (N. Ashcroft, PRL 21, 1748, 1968) which addresses the topic of metallic hydrogen and density thereof, working to the conclusion that metallic hydrogen may be a high temperature superconductor. This was actually cited as a possible justification for the large magnitude of the magnetic field of the planet Jupiter. I don't think that answers your question, but I think it's pretty neat. :)