Hi Dan,
I'm also on livemocha, which means it isn't a case of choosing one or the other. While LM has more structured language courses and actively encourages members to help each other, I enjoy the freedom of italki, where what you write isn't limited to the exercises provided. Apples and oranges, I think. :)
Also, you couldn't ask a question like this on livemocha. No "Answers" section there. ;)