Bem just nailed both of the scenes that he was in. He could have come back in later movies to talk about the Azkaban breakout in OotP or horcruxes or the Elder Wand in HBP and DH. Or about the deaths in past Triwizard tournaments in GoF.
I'd say both should have had more screentime, but I'm going with Astoria, since she ended up being Draco's wife, and I think knowing a bit more about her would have been nice.
