I believe the novel mentions that Tessio was more cunning than Clemenza (who was more direct and brutal; I believe the novel also mentions that Clemenza didn't like guns and preferred the garrote). The original reason why Tessio and Clemenza were split up is so they couldn't conspire against Vito but also there was a line mentioning that many families thought Tessio was acting on his own and wasn't affiliated with the Corleones.