it is better for the women who is going to marry to uncover the face for the witnesses when doing the nikaah or to keep the face covered?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

A woman must cover her face from non-Mahram men according to the most preponderant opinion of the scholars. This is applicable whether during the marriage contract or in any other situation, unless there is a need for her to uncover her face, such as when the witnesses should see her [in order to ascertain her identity] if necessary, if she is not known to them. In such a case, she must expose her face because it is not permissible to accept their testimony regarding her (that she is the one getting married) while she is wearing the Niqaab (face covering).

There is no difference in this ruling whether the testimony is related to her marriage or other transactions, as the scholars stated.

Allah knows best.