Y'know, I've heard some people are filling the insides of their guitar with air these days...
I have some thoughts on this, though it's admittedly no more than theory. A good acoustic guitar is built on the principle that it should have no more structural stability than absolutely necessary. Production guitars tend to be overbuilt, so people that don't know better are less likely to break them (read: cheaper warranty expenditures). Luthiers tend to get rid of as much wood as they can while still maintaining structural stability.
Electric guitars are built on a slightly different principle. The solid body was conceived (as I recall) to eliminate feedback, which had been a problem with electrified acoustics. In terms of structural stability, they are generally WAY overbuilt, because they have different goals.
As solid bodies incorporate denser materials, such as aluminum, granite, and the like, people have reported greater sustain. The way I understand it, sustain is achieved by tighter resonant frequencies that hold out longer, and seems to be encouraged by denser materials.
So to say a softer material is more resonant than a dense material can be misleading, as the resonance may just work differently. It's like relaxing string tension, isn't it?
I may be totally off-base with all this. I haven't tested it, and maybe some more experienced builders can chime in. Regarding Balsa wood, I'm all for trying something new. That's much of the point of building your own guitar. But while softer woods may gain you something in some areas, there's probably an opportunity cost.