Democritus vs John Dalton atomic theory
Demokrit was a philosopher, while Dalton was arguing on the basis of (somewhat limited, but still) solid, quantitative evidence. I'm not knocking Demokrit (!), but his was not a scientific statement, let alone a theory. You cannot compare the two, or at least you should not try to compare them from the view of natural science.

Both came to a similar conclusion, but they were reasoning on a completely different basis. Of course, Dalton had the advantage that the idea was already there and known to him.

In Dalton's time, the modern concept of elements was already known and widely accepted. Iron, oxygen, etc., not fire, water and stone. You could say that he knew what he was talking about, regarding atoms. And Dalton and his contemporaries had the numbers they measured.

Demokrit, on the other hand, had no idea about quantitative elemental analysis. Which ultimately was the downfall of his idea: He had no numbers that others could have proved and extended, or disproved. Only when the chemists at Dalton's time were able to make quantitative measurements the atomic theory could become useful. Demokrit had probably theorised that atoms of the same kind had the same weight (we don't know exactly, because we have very little original records from him), but Dalton now had solid evidence, that only made sense if that was indeed the case.