Bluetooth is the wireless link between the PC and the saber module itself. It sends data to the PC so it can figure out which sounds to play out (swing, clash, lockup, force push, etc).
The onboard speaker is a different board version, the bluetooth version relies on an external sound source (stereo, PC speakers, PA amp, etc) to play out the sounds. This is the more professional setup that guys like Ben Burtt, Elstree, and saber demo practictioners use in their line of work.
I dont have an onboard speaker selected yet, but it will be a full range speaker. The basic downside is loudness and sound quality when you go to an in-the-hilt sound board with speaker.