the diff in 6 inches… we go the smaller one. the 18×42. the bigger one is 18×48. i been in one of those before. it’s still filling up with water. been filling up since 2:30 this afternoon. hope to swim tomorrow… they didn’t have the bigger one where we bought it. just the smaller one. but it was the biggest at the store… of course i had to compare them though lmao.