Dragons are/were the predecessors to the Diablos. They are pretty old tyres. They were replaced by Dragon evos before the diablos came along to. Somebody has more than likely picked the pair up in a sale for not many £'s. They were pretty good in their day, but they don't come anywhere near todays tyres i'm afraid.
The honest answer is that they are the tyres for me. They suit my bike & my riding style. Tyres are a peculiar subject and everyone has their own favourites. What I would say is that if the Dragons you have are ok for you, then you'd love the Diablos. It'sall down to choice, but if you decide to gowith them I doubt you'll be disappointed. Andy