Not my field, but it seems obvious that this is an issue of hierarchy or local~global scale.
If you act out of an individual or local judgement about consequences, then on the plus side, that allows creativity of response, but equally, you are making guesses it will work out.
If you follow rules, then these will be global habits that have developed over time (as the result perhaps of many local guesses) and will have proven their general worth.
So an ideal system would have the two kinds of utilitarianism in balance. You want some degree of local creative freedom of action - act u - so that society can experiment and learn. What happens for example if you come across novel situations for which no global or general rule is available? Individual trial and error is how a new rule would eventually arise.
On the other hand, societies would represent the long-run knowledge of what works out best. The accumulated and distilled wisdom. So rule u should be the general approach, the global context of personal action.
As usual in philosophy, you are being asked to chose a side. Option a or option b. And as usual, the answer is that when two options both seem strongly reasonable, it is because they form the necessarily complementary aspects of the one greater system.
Oh, Wiki says...
A problem with act utilitarianism is that people never know what acts to expect from those who practise act utilitarianism. Arguably a greater number of people are happier a greater part of the time if they can trust others to follow standard moral rules most of the time and know what to expect.
That is probably so, but hardly a major issue. It of course also seems to apply that living in a society where everyone sticks to the rules and never shows any creative latitude is just as bad. Ever dealt with one of life's bureaucrats?