The objects described are different: diffuse flying describes the way light objects such as snowflakes, leaves, petals, etc. float and flutter in the air. Rambling: describes the way heavier objects (such as flags, smoke, etc.) flutter and fly in the air with the wind.
Gives a different feeling: diffuse flying focuses on describing the state of objects dancing in the air, giving a light, colorful feeling. For example, when autumn comes and the weather cools down, the yellowed leaves are flying in the sky. Flying in the sky focuses on describing the state of objects flying in the air with the wind, giving a feeling of floating and spectacular. For example, in the wind, the flag is flying in the sky.