This is up to you. Fruit in syrup is sweeter and tends to have a creamier consistency once processed than fruit in juice. Use what you prefer to eat. If you like the fruit in the syrup, use that. If the fruit in the syrup is too sweet for you, then you can use fruit in juice. Keep in mind that fruit in juice does not turn out as creamy. But, for this recipe, the bananas ad a lot of extra creaminess.