As I recall, all the dishwasher instructions I have read indicate that
the main detergent goes into the covered dispenser, where it will be
released for the main wash cycles. The uncovered cup is for optional
additional detergent for the first cycle, to be used if dishes are
extra dirty. If you only put detergent there, it will likely be gone
before the main washing cycles. Plus, at least on some DW, the second
one is smaller than the main.

As far as merits of tablets, powder, etc, Consumer Reports testing
concluded that the main difference with the ones that did the best job
was that they all contained enzymes.