This is tricky. At first you might say that LMAO is more crude, but then rationalize it by saying that the A is meant to be a donkey, and not a human rear end. However, we know this isn’t the case, and that when most people use LMAO they’re referring to themselves, not their pet donkey, if they even have one. Besides, if that was what was intentioned, it doesn’t work, since you’re saying M “my”. How can you laugh your donkey off? It doesn’t work if you’re trying to be cute, only if you’re trying to be sharp or witty. And since most people aren’t trying to be cute when they say LMAO, we can conclude that it is the more inappropriate of the two, if given a choice.
LMBO may refer to the same thing, but B is less severe than A. It ends up sounding childish and an obvious attempt to say a watered down, G-rated version of the same thing.
If all else fails, create your own lingo and see if you can get it to catch on.
Like maybe LMS (Laughing Myself Silly). It’s shorter and isn’t inappropriate at all. If you do use it and it catches on, make sure to give me credit for it, ok?