Gibbon and Darwin are both great writers. However, the cheering Gibbon gives to the persecution of Christians by Marcus Aurelius (in other respects a great man) is pretty shameful. His theories about the fall of the Empire are original and true, but where they are true (the loss of civic virtue) they are unoriginal and where original (Christianity killed the Empire) they are not true. Still, Decline and Fall makes for great reading, as you say. I wish "Gladiator" has stuck closer to the facts therein.
"The Fall of the Roman Empire" was much better than its remake, "Gladiator." Better cast, better music, better direction, more accurate, etc.