Is it better to save the money designated for printing stickers that say "Muhammad, peace be upon him, is in the heart of every Muslim" and donate it to the poor, or to print and distribute these stickers?

The merit of a deed follows its benefit and the people's need for it. If the stickers remind people of the Prophet, loving him, and following his Sunnah (definition), and no one else is doing that, then it is a righteous act for which one will be rewarded according to the sincerity of intention and the outcome of the deed. All deeds are weighed against each other in terms of merit and reward according to the time, place, the doer, and the extent to which the benefit of the deed extends. Charity and spending on the poor are among the best and most greatly rewarded acts of righteousness. It is preferable to combine both good deeds. If one of them must be chosen, then you may choose whichever is more beneficial, useful, and rewarding.