I have heard that iflight's Green Hornet uses T700 carbon instead of 3k carbon. What difference does the new T700 carbon make?
First of all, the 300 or 700 is the grade value of carbon fiber and generally, a higher grade means more strength and better performance. The main differences between T300 (3k) carbon and T700 carbon are that T700 carbon has almost 40% more tensile strength than 3k carbon, T700 carbon is slightly denser (and thus heavier for the same volume of carbon), T700 carbon fiber has a higher carbon content and a lower nitrogen content, and T700 carbon fiber is smoother looking while T300 carbon looks more bark-like.
So, overall, T700 carbon is better for making a drone frame because it is significantly stronger with only a small increase in weight.