Both of these cards will become Tier 1 draw supporters in the Black and White-on format. They have similar effects, allowing you to draw several cards directly from the deck, but they do have differences.
Cheren can draw 3 cards with no drawbacks and no risks. Bianca, however, can draw more or less cards depending on how large your hand is. After a late game N, Bianca is much better because you can draw 5-6 cards and most likely win the game. Cheren would also help, but to a much less extent. Cheren would help, but it probably wouldn't allow you to win that turn. Earlier game, Cheren could be more helpful because you may have a larger hand and N gives more cards, but Bicanca could be more useful after you use Ultra Ball or some other cards. What do you guys think is better and which would you play more of if you ran both in a deck.