Co 2 Cylinders

[pillairp]

in market there are 2 types of Cylinders for Co2.. Aluminium & Carbon Steel, please which one would be good??

[rich311k]

Does not really matter. The aluminum will not rust.

[dbosman]

The metal tank weight is less with aluminum which may matter for shipping.
The other concern will be your CO2 supplier.

Some places, such as fire extinguisher shops, refill cylinders/tanks/bottles.
Other suppliers, only swap. They ship empties to a central location for refilling and testing as needed.
Aluminum is pretty, but if your supplier only swaps, you won't have it all that long.
By the way, gas cylinders have to be hydro tested every five years. The date of that test is stamped into the metal. If you swap tanks, you'll never have to be concerned about the test date or paying for a re-certification.