All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
Condolences is not an occasion where one sits and discusses and so forth, what is legislated in it is to visit them and ask Allaah to multiply their reward on the calamity which befall them which is the loss of their beloved one, and ask Allaah to forgive and have mercy on the deceased.
As regards the marriage banquet, if you are invited to it, you are obliged to attend as the Prophet said: "If anyone of you is invited to a marriage banquet, he should accept the invitation and attend." [Al-Bukhari and Muslim]
Therefore, it is more appropriate to combine between the two things, but if you are unable, then attend the marriage banquet, as the duration of condolences is three days, so go for this purpose in one of these three days.
Allaah Knows best.