Simple put, USB (upper side band) and LSB (lower side band) are parts of the same signal. An AM (amplitude signal) consists of "the carrier" with any modulation on that carrier creating the 2 sidebands, one upper and one lower. A SSB (single side band) signal has the carrier and one of the sidebands removed.
The 2 main advantages being that the tx is much more efficiant and the signal much narrower taking up less bandwidth so getting more stations in a given space.
As to which to use it does not matter but BOTH the tx & rx need to be on the same one
Hope this is of help.
Tig.