RE: Cheren vs. Bianca?
I hate Bianca, Cheren wins, 'nuff said. OK, I'm actually going to try to be helpful here, but I may be overly biased occasionally.
With Bianca, drawing it early game is almost always the worst. You usually have a hand full of resources, like Catcher, Switches, Energies, Computer Search, evolutions, and stuff you really don't want to get rid of. If you have Bianca in your hand, you have 2 choices: Use a whole bunch of stuff that's going to be useful later and maybe draw like 3 to 4 cards. Or you play as conservatively as possible and draw like 1 or 2 cards. Both of them put you in a terrible position to win the game, and Cheren is just straight better in that situation.
Sometimes you get those hands that are full of search cards and stuff that's good to play down, and you usually still just draw around 3 cards. Which is still worse then Cheren, since with Cheren you have knowledge of what you're going to have before you play those cards.
Now, the only perk of playing Bianca in my opinion is it's late game power over Cheren. In the late game, after you've used resources for more productive things, and N after N makes your hand smaller and smaller, Bianca because more powerful the later the game goes on.
If I were to ever play Bianca (which I probably won't), I'd probably only play around 2. Having the standard 4 Juniper, 4 N with your only other supporters being 4 Bianca is probably the dumbest thing I can think of doing in a deck. You want as little possibility as possible of having it in your starting hand.
Pretty much the only deck where Bianca might be better then Cheren in a lot more situations is Darkrai decks, which have Sableye to get back those trainers. But even Darkrai can have hands full of unplayable cards (like Hydreigon pieces, Eviolite's but no Darkrai, Potion), or no Sableye to get them back for a while. But all of this is pointless since Darkrai just uses Skyla and Random Receiver now.
I also don't like Skyla for similar reasons. Where half the time I draw it, I'm going "Why can't this be ANY other Supporter?!" But with Skyla, it's more based on the deck. Bianca, it's like that in every deck.
Neither are that great, I'd usually just use Skyla and Random Receiver instead of both anyway. I think the choice is based on whether you want to risk starting with Bianca, or risk having slightly weaker hands with Cheren, and I do mean slightly.