Stereo = encodes each l/r channel seperately with full bitrate, but is not as efficient with bits as JS
Joint stereo = able to allocate more bits where it needs them, due to encoder throwing away monotonous information that is shared across both channels
I would guess that Force Joint Stereo... forces joint stereo. Dual Channel, I don't know what they are referring to.
Joint Stereo is the one you want. It allows more bits to go to the channel that needs it the most, and consequently, your sound quality should improve, theoretically.