I'd assume that the main difference between the two would be their roles/armament. From looking at the concept art, the Sparrowhawk seems to boast a powerful arsenal of anti-tank weaponry, but no machine guns or other anti-infantry weapons. The Hornet seems to be a bit more geared towards anti-infantry roles.
Alternatively, the Sparrowhawk could be an uberunit/hornet on steroids.
I asked The Lord, "Why hath thou forsaken me?" And He spoke unto me saying, "j00 R n0.0b 4 3VR", And I was like "stfu -_-;;"
Check out my ongoing FF, Age of Revelations.
[[I approve of the firebase]]