Hi and Welcome to the DISboards
I only have experience with the Soda Mug. Yes it is pay one price and have as much as you want.
Is it worth it? Depends on how much soda you drink. It is free with your meals in the dining room. If that is enough for you then no it is not worth it. If you like to drink a lot during the day and the weather is quite warm then yes it probably would be worth it.
I think now that you don't have to buy the Mug, you can just get a sticker to put on your card and they just give it to you in a glass.