This is not a good answer. Rock slide is the clear victor. If you think logically, aggron is a hard hitting pokemon, that relies on strong attacks to take out pokemon quickly. The speed drops that rock tomb would grant you are useless. In the time that it would take for you to effectively get enough speed drops on the opponent to out speed them, aggron could already have defeated the opponent with half the amount of rock slides, as while you falsely stated that rock tomb had better accuracy, it doesn't (rock tomb 80, rock slide 90, this is GEN III), and it has 50% more power, 75 over a mere 50. Rock slide is the obvious choice.