Please help to clarify whether both sentences "it's good seeing you" and "it's good to see you" are correct or not.
They're both correct; but 'it's good seeing you' puts more emphasis on the event (the whole process) rather than the first sight. So when you see someone you might say 'it's good to see you', and at the end of the encounter you might say 'it's been good meeting you [after all this time]' (and afterwards you might send an e-mail saying 'It was good seeing you [we really must see each other more often]'.
So although they're both acceptable, the "to" one is possibly more common (at least with "is/are" , and the verb
see). But it really does depend a lot on context.
b