As the link says, the terms are interchangeable these days. I think it's matter of preference or perhaps regional differences as to which is used. I think I use couch 99% of the time. But I wouldn't be surprised to hear somebody say sofa.
I agree with Lothor. I think couch has gradually become accepted in British English. You're right we certainly would not say 'sofa potato' unless we were playing with words / having fun with the language! Some people like to take a set phrase / idiom and either shorten it - Too many cooks! or change the ending to something else. However you need to know the language and your audience well to do this! So it comes with a health warning!
If I can be of any more help let me know please hirashin.