Fuzzy nailed it.

I built two bolt ons with similar necks - same LP Jr-ish body shape.

One is semi-hollow (actually very hollow, almost to the point of structural instability.

The other has no control cavity (one pick up guitar, notvolume or tone) . Only a bore drill to the pick up cavity from the jack

All of the below can be heard acoustically from each guitar, without any amplification:

The hollow one does not have the constant sustain of the solid one (the note drops off in volume quickly, but it sustains a similar length of time). There is a delay to the attack, almost like the body generates the sound and then passes it to the string. The hollow is far more reponsive to "touch" - lighter fretting or picking make a BIGGER difference with this guitar.

The solid one has better volume of sustain, immediate attack, etc.

One isn't really "brighter" than the other - so pick ups, assuming like quality. will not adjust the sound for the semi-hollow.