Which is better and more prioritised: For a Muslim to give charity with money, or to use it to buy something that enables him to act upon the Sunnah, such as buying a siwak?

It is best to combine miswak and charity if possible. If not, and the charity is to save a life or feed a hungry person, then charity takes precedence over miswak, because satisfying hunger and clothing the naked are more important than purifying the mouth. This is also because the benefit of charity is transferable, while the benefit of miswak is limited. The Sunnah has indicated the precedence of benefits that are transferable. However, if the charity is not for someone in need, or if neglecting miswak would cause harm to people, such as bad breath, then miswak takes precedence over thendiscretionary charity.