They're both couches, only one's got four letters, one's got six. No, no difference as far as I know, just (as gary said) depends on where you come from (or, maybe, where you're going)
A settee is a seat with a back and sometimes arms for two or more people. A sofa is an upholstered seat with a back and arms for more than two people. A couch is a piece of upholstered furniture usually having a back and armrests for seating more than one person. Collins English Dictionary.