You'll want to use single
quotes where you want double quotes to appear inside the string (e.g. for html
attributes) without having to escape them, or vice versa. Other than that,
there is no difference.
I wouldn't say there
is a preferred method. You can use either. However, if you are using one form
of quote in the string, you
might want to use the other as the literal.
alert('Say "Hello"');
alert("Say
'Hello'");