The main difference is the direction the flower head faces.
Scilla have electric blue flowers that are bell-like, drooping downward.
It's six blue petals surround stamens that have thread-like filaments and are not clustered together. Scilla can grow 3-6 inches high and each bulb produces 3-4 stems with medium, strap-like leaves. It's very tough and cold hardy that easily naturalizes by bulb offshoots and self-seeding.
Chionodoxa is commonly called Glory-of-the-Snow because it is among the first bulbs to bloom in the spring. Each bulb produces six petaled, soft blue flowers with white centers that face upward and are surrounded by narrow, basal leaves.
If you look closely, you'll notice the stamens have very flattened white filaments arranged close together, giving the appearance of a central white cone. The plant naturalizes easily by bulb offsets and self seeding.
Both these bulbs are planted in the fall and during the late winter/early spring months begin to bloom. They may be small, I think they are both adorable and beautiful!