The pros of iMessage are that they can work on iPad and that they deliver always. If the message does not succeed delivering as an IM, it defaults into SMS after 3000 seconds. The cons are that they are only iOS compatible and that they are suitable for text and pictures only. The pros of SMS are that they are a cheap means to communicate and that they are convenient and easy to use. The cons of SMS are that they may not give full information as one would require.