The difference between our co2 tanks is pretty basic. The larger the tank the more Co2 you can fill into the tank so the more shots per fill. The only other difference is is the material they are made of. Aluminum tanks will be lighter and Chromoly tank will be more durable but heavier.
Our tanks ship empty. These can be re-filled at sporting good stores, paintball fields, and many other places. Some Walmart and Kmart, Sports Authority stores also fill these tanks.