Which is better: performing Hajj or marrying a second wife, in the event of not possessing sufficient funds for both, while already having a wife and children?

Hajj takes precedence over marriage for someone who has not yet married, unless they fear hardship (al-'anat), in which case marriage takes precedence over Hajj, because then they would not be considered able to perform Hajj. As for someone who is already married, Hajj is obligatory for them before marrying a second wife, unless there is an urgent need for them to marry another, in which case that takes precedence over Hajj.