"Stereo" is full stereo, as the source material is intended.

"Joint Stereo" is actually a misnomer, because some of the audio common to both channels is downmixed to mono to save space. It may improve (subjectively) the sound of material encoded at <= 160kbs. Unless you are trying to store a gazillion songs and don't care about quality, don't go there.

Stick with higher bitrates and "Stereo" encoding (256 kbs - 320 kbs is good for MP3), and you won't be disappointed.