co2 tanks, steel or aluminum

which should I get, is their a differance, i know steel is heavyer but by how much, iv allso heard that a lot of places will just take ur tank and exchange it insted filling so its not rly worth paying extra for a aluminum one

Well shipping costs for one. If you can find a reliable seller online (cough cough.. PM me..) You will need to pay to have it shipped. Also depends on the grade of steel. If you get a stainless steel tank it will be more resistant to humidity and weather than a cold rolled welded chinese steel tank. Aluminum is great too bacause its naturally resistent to weather. but cant hold pressure tolerances as well as steel.
Personally I use a gen 2 co2 burner and just get LP tanks refilled instead. Its slightly cheaper but the real reason I go this route is becuase when LP is converted to co2, Its in a gas form which is easily accepted by your plants. When you release co2 from a co2 container. Its in its liquid to vapor form which is more dense and has less surface area then co2 that is created from burning propane. Hope that helps