Which is better in the view of Islam: building a new mosque that will not be frequented by worshippers for many years, or spending its costs on orphans, widows, the poor, and the needy?

Preferring some acts of worship over others requires a legal text (nass shar'i) or a general rule, and personal preferences have no place in this matter, just as the aspects of merit are numerous. Building mosques is preferred over other types of charity, because it is an ongoing charity (sadaqa jariya), and aims to build for the hereafter by establishing prayer and remembering Allah, whereas other charities may be more immediate in benefit but their reward ceases. It is not possible to definitively state the superiority of one over the other in absolute terms, for there is good in each.