What all the comments boil down to is we are comparing different things:
Allyl vs benzyl cations: Both contain onlybhydrogen and carbon, with the positive charge formally distributed over two carbon atoms in allyl versus four carbon atoms. Putting the positive charge on more carbon atoms makes the benzyl cation more stable.
Acetate vs phenolate anions: Here, unlike the hydrocarbon actions mentioned above, oxygen enters the picture. The negative charge in the acetate ion is distributed over two oxygen atoms, versus phenolate having its negative charge on one oxygen plus multiple carbons. Having the second oxygen atom wins out because of the superior electronegaytivity of oxygen, which is a nonfactor for the cation case mentioned above.
Note that the "rules" above are not absolute laws. You have to consider other factors such as aromatic or antiaromatic coupling that involves the charge; e.g. cyclopropenyl cation is much more stable than benzyl cation because of the positive charge being inherent to an aromatic ring, even though cyclopropenyl has only three instead of four carbons to take up the charge.