It is preferable to have a small but certain advantage than a mere potential of a greater one.

Etymology

Earliest known English usage in 15th century in The Life of St Katherine by J. Capgrave. Used in 13th century Latin (Plus valet in manibus avis unica quam dupla silvis "...in the woods", rather than "...in the bush"). However, a much older Near Eastern origin is suggested by a near equivalent in the 6th century BCE Proverbs of Ahiqar: 'a sparrow in thy hand is better than a thousand sparrows flying'.

Sources

Wiktionary