What are the main differences between Bentham and Mill's utilitarianism and which theory is better?

Both thought that the moral value of an act was determined by the pleasure it produced.

Bentham considered only quantity of pleasure, but Mill considered both quantity and quality of pleasure.

Bentham's utilitarianism was criticised for being a philosophy "worthy of only swine". This is because he made no distinction between the pleasures experienced by beasts and those experienced by humans.

"Quantity of pleasure being equal, pushpin is as good as poetry".

Mill acknowledged this, and to sidestep the criticism, he considered both quantity and quality pleasure. Mill distinguished between higher pleasures (those that require mental faculties that only educated humans could obtain) and lower pleasures (bodily pleasures that both animals and humans could experience). For Mill, higher pleasures are more valuable than lower pleasures, because of their "intrinsic superiority".

Though Mill's theory is more respectful of human nature, it makes pleasure even more difficult to calculate as we now have to consider unquantifiable quality of pleasure, as well as the quantity. Bentham's theory was act utilitarianism, but Mill's was rule utilitarianism. Bentham's theory applied the principle of utility to individual acts and situations directly. This meant that some abhorrent acts were permitted.

For example, two torturers may be justified in their activity if their pleasure outweighs the harm done to the victim.

Mill developed rule utilitarianism to avoid this. Mill suggested that the principle of utility should be used to determine moral rules which govern utility. E.g. Do not kill people (as killing people tends to lower net utility). This seems like an improvement, but there are situations where breaking the rule increases utility - where it may be expedient to break them, to put it Mill's way.

Not to do this is to worship the rules rather than the principle of utility itself. In order to avoid rule worship, the theory collapses into act utilitarianism, as we have to make exceptions to every occasion of this sort.