For a clear or translucent finish, a single piece of lumber might be gratifying. Otherwise there's a good chance you'll get a piece with a visual interruption in the grain pattern at the glue joint. For a painted body, there's no reason to prefer one over the other.
Some folks say heavy bodies yield better sustain. I say with very rare exception, sustain lies in how well your nut and bridge function, and how your amp reinforces the string vibration, not the weight of your guitar. I've played boat-anchor guitars with crappy sustain and featherweights that sang for a long time. YMMV.
The conventional wisdom suggests mahogany will be "darker" sounding, whatever that means (probably some of the treble frequencies tend to be dampened by the body). But in my view, this is much less an issue with your body than with your neck lumber, since the neck is actually more likely to absorb resonant frequencies than the big, inert chunk of wood that makes up the body. This is not to say your choice of lumber will have no bearing on your sound, but I would say the most important choice for your tone is your electronics/pickups; then neck shaft wood; then hardware choices for bridge and nut, and finally body and fingerboard in about equal measure. Others will likely argue, and my own order is not necessarily a dogmatic view on my part. More of a "here's how I reckon it likely shakes out," rather than "This is The Truth."
