There's more than one consideration, and weight is usually only an indicator, not a rule. Generally speaking, the denser a wood is, the more it's going to sustain, while the harder it is the more it's going to bring out the higher frequencies. Density and hardness usually go together, but not always. It depends on the species. For instance, balsa is a hardwood, but I can't imagine anyone would call that heavy or dense... quite the opposite. The stuff's almost like styrofoam <grin>
Most of the qualities of what are considered "tone woods" are really only apparent in acoustic guitar construction. You have to go to extremes to get those qualities to show up in electrics. Part of that is because there's just so much mass in an electric. It's not going to absorb the energy in the strings and resonate in sympathy the way an acoustic will. On the other hand, electrics get heavily amplified and the differences are amplified as well to become more obvious.