Both models are for stretching the perineum and preventing tears. THe Delphin Plus has an additional manometer attachment that allows you to practice strengthening the pelvic floor muscles, especially for after the birth. The manometer gives you feedback on the effectiveness of your muscle squeeze (kegal exercizes).
Obviously, it is best to buy the Delphine plus and do the pelvic floor exercises after the birth, from about 6 weeks after. However, if you are the type that won't do it and it will just sit in the box, then it is a waste of money, as it costs an extra 120 NIS.