Sigh... I keep mixing up the word for dress and clothes. Practice, practice, practice!
En klänning = a dress (en-word)
Klänningen = the dress
Klänningar = dresses
Klänningarna = the dresses
Sadly, these are not interchangeable.
Both are acceptable. They're pretty much interchangeable except "gillar" might be a teensy bit more casual, which is probably why a lot of Swedes told you that's what they use.