Gonna go with the underdog this time, so Iris. I don't know how different they are in battle in B/W, their teams are almost identical though. Thing is, I really like the history behind these two; the spartan elder who trains with his Dragons barehanded, travels the world to find a worthy disciple. He arrives in the Village of Dragons, where he meets a prodigious young girl*. He takes her back to Opelucid City, and trains her to become the new Gym Leader. It just makes so much sense, so when I look at Iris, I see a strong Trainer, not just a kid -- the anime didn't do her any favors in this aspect though. Plus, age shouldn't matter, the protagonists themselves are very young as well.
* all of this is actually mentioned in-game, via the Memory Link feature in B2/W2, I'm not making stuff up :P