Wie sagt man das auf Spanisch (Spanien)? 1. No pareces muy convincente.
2. No pareces muy convencido.

Cuál es la diferencia entre ambas ?

Sehen Sie eine Übersetzung

@marcoh the option number one means that you are not bealiving what the other person is saying because what he/she is saying seams very confusing like the argument doesn’t match with the body language or something similar. An untold doubt from the listener to the speaker about the truth of the argument remains.

And the second option means that the person who is telling the argument doubts of himself, maybe he/she is not very sure about the decision he/she already made or is about to make and the listener notice it and then make the statement.

Both sentences represent an answer to something but the difference is who is doubting about the argument. Sentence #1=listener doubts.
Sentence#2=speaker doubts of his own argument and the listener can notice.