Is it legally preferable to postpone Hajj and donate its costs to those in need in Syria, or is Hajj more obligatory, given the ability to perform it this year?

If neither you nor your wife has performed the obligatory Hajj, then Hajj is incumbent upon both of you this year if it is feasible, because it is immediately due. The obligatory Hajj is more binding than financially supporting Muslims, as it is one of the pillars of Islam and must not be delayed. However, if your Hajj is a supererogatory one, then donating money to support Muslims in Syria is better than performing a supererogatory Hajj.