I don't know where in the country you are located but Florida requires prefabricated steel reinforced concrete tanks including a concrete lid with an access port for clean-out that has a recessed attachment point to lift the access port lid out. The entire unit is lifted, set in place and leveled with a front-end loader. As far as which is stronger the better question would be which one will last the longest. Steel will rust and become weaker over time while steel reinforced concrete takes about 45 years to reach its maximum hardness before starting to decline.
I believe a concrete closing for the tank will be better. This is because it will be able to be firm and since it won't be likely moved, will last for long. If you wanted to move it, you could still do the same by screwing a metal chain to the concrete for access of the septic tank.
Most septic tanks come as a complete tank. They have a cement top which is sealed with epoxy to prevent water from running in from the ground. They have a lid that is removable so it can be inspected and pumped. They have holes in the sides for the plumbing to connect.