I would say that Sidious did not intend to lose Maul and need to recruit another. He therefore did not have a specific, unique purpose for each.
He trained Maul from youth, forging him into a powerful Sith over the course of years. However, at the time of Maul's death, it was too deep into his plan for Sidious to train another apprentice from such a young age as he needed one very soon. This is why he chose Dooku. Dooku was perfect because he had already trained with the Jedi for decades, and he had also left the order and begun to experiment with the dark side, so he already had some knowledge in that area as well. This is why Sidious chose Dooku.
I honestly Think Dooku was as powerful as Sidious. He took on Anakin and Obi-Wan twice and held his own rather well. Kenobi is one of the greatest Jedi during his time.
Then you have the Clone Wars cartoons in which he plays with Grievous and Ventress.
I really think Dooku could give Sidious a run for his money.