CO2 tanks are cheaper, but you have to pay extra for fills at some fields. Though, CO2 can last longer if it's warm enough. HPA tanks have regulators and air doesn't change phase with pressure or temperature so the output pressure is more consistent. The pressure and temperature of CO2 will vary at higher rates of fire, so accuracy can suffer and if the gas gets too cold or liquid CO2 makes it into the gun it can freeze your paint and then chop it.
Basically, HPA is better so if you can afford an HPA tank go with that.
like most people said using the search function on this page will get you a long way when yu have a question. with that said, i will literally give you everything in my paypal account (currently around 12 dollars lol) if there is someone who has a valid argument as to why you should use co2 over hpa... hpa ****s on co2 in so many ways... not to mention that once you get REALLY into paintball and buy an electronic gun, co2 will destroy tat gun to the point of no return.