Giovanni,
If we assume for the moment that it will be built to a structurally sound and therefore safe design, there might be two. The first would be material cost. Copper of the size and weight you quote is incredibly expensive so in the size you quote steel would be the material chosen by most people on account of cost alone. However, if you somehow got the copper very cheaply or at no cost then that's not so much a factor. Also, if copper of far less thickness than you have or propose is used then one could add strength as a factor to be considered.
The second consideration would be your individual circumstances, your resources, abilities, and equipment. What you have and are capable of can tip the balance one way or another. For instance I am an abysmal welder but a very good silver-solderer so I build almost exclusively in copper in spite of rising material costs. Some people are very good welders, or have access to reasonably priced welding services, so it makes more sense for them to use steel. It takes a great deal of heat to solder the boiler you propose, so much so that it will be uncomfortable standing too near the boiler while it is up to full soldering heat, and most amatuers don't have the heating gear needed to do the job. The cost of acquiring such gear, and the likelihood of using it again, needs to be factored in the balance sheet.
