briefly , in chinese there are two words with the same pronouciation , "坐" and "座" . 坐 means "to sit" and 座 means "seat" ( notice the common structure ) . and the word 请 ( as of modern use ) means "allow me let you do xxx" in a polite manner .
so 请坐 is literally "allow me let you sit down" but is generally interpreted as "please sit down" . and 请就座 is literally "allow me let you move yourself approaching some seat" i.e. "please take seats/a seat" .
more often in daily conversations we say 请坐 .
a typical usecase 请就座 is , when the speaker is host of some gathering , and plural people are entering through a door , the speaker may say 请就座 after greeting them . this sounds more formal .
both terms are used not affected either for lower class speaking to higher class , or for higher class speaking to lower class .
though syntactically by 请坐 the we assume the listener(s) is(are) close to the seat(s) and by 请就座 we assume the listener(s) is(are) away from the seat(s) , this is not usually a big factor of how we determine which phase to say .