There is no difference between those two phrases, but the first phrase is more common. And "(a) kako си ти", can be the answer/reaction to the question "Kako си?". The situation : Здраво, како си? Хвала добро (а) како си ти ?
N.B. The pronoun ТИ in the case of a text, can be written with the capital letter as a sign of the rescpect. (Ex.: Драга бако Ти ме увек изненадиш и Тебе се увек присетим ...
Good luck !