Cheren just seemed more of a rival to me. Before I battled him, all the way through the game, I checked the levels of my pokemon, made sure they had good moves and put them in a good order.
With Bianca I just powered straight through, like it was nothing.
In a rival in the games, I prefer something quite difficult to work with, and it seemed Cheren was a better fit for me than Bianca.
Also, Cheren is a lot like me, doesn't think much about the future, only about the task he's trying to complete at the time. Bianca thinks too much.