Even though Atlantis may have some better tools, Silk Road has a very loyal base so it will be very hard to switch users over. The first mover in an industry often has a big advantage and in an illegal one such as this it will hard to convince people to change. The users on the Silkroad forums are very active and that makes it a strong community.
Most people that have visited both sites will agree that Silkroad has a far better selection of products. Most merchants that have accounts on Atlantis get just a handful of orders a month. It comes down to trust. Silkroad was set up and grew organically while Atlantis’s self promotion smacks of desperation. Who knows who the owners of Atlantis are, they may have been set up by Feds to trap dealers in a sting down the road somewhere. It all seems a bit fake and phony.