Language / Difference between 'to' and 'jest' when describing people [4]

Hello,

I wonder if someone can tell me... what is the diference in meaning between:

Kasia to mila nauczycielka

-and-

Kasia jest milą nauczycielką

(I've learnt that this one is the instrumental case, to be used with expressions like 'Jestem Polka' and 'Jestem Lekaerzem').

I'm Confused!

Thank you :)