Hi,
It depends how often you consume it.
Diet coke is a beverage that contains artificial sweeteners instead of
sugar which is even more harmful.
Moderation is the key. You can have it once in a while but it shouldn't be the regular consumption for you.
Regular consumption of diet Coke may have certain harmful effects on you like weight gain, tooth decay and dehydration in the body in the long run.
Also, with the fruit juice, it's better to have fruit instead of a juice so that you store the fibre and the nutrient contents in it well. Again the same rule, once in awhile and in moderation you can absolutely have it !!