The difference between each machine is relatively small, you can take a quiz here which will help you decide which machine is for you and why. Or you can also find a comparison chart on each TENS hire product page.
Though, in short, the differences are:
Elle TENS - Base model, has birth mode for pain relief in labour
Elle TENS 2 - Same as above, but also has the screen light and contraction timer
Elle TENS + - Same as above, but also has the standard pain mode which is helpful for pain relief outside of labour (great for afterbirth pains)