I'll comment differences in the two.
In BK's the giant Grunty is reaching for the titular characters. A nice, simple effect that evokes a sense of immediacy.
YL's is more complicated -- Capital B is laughing maniacally as books are swooped up around him; and YL are instead chased by minions.
While YLs has a creative idea and says more about the plot, in BKs there's a greater sense of cohesion and that immediacy.
YLs is more spacious and had this fantastic, tasteful painterly style. (It's nice Playtonic has an artist like Dean who could work for Disney or Pixar, or work for the Vatican and paint portraits of different personages.)
BKs box art is cluttered in a charming, grizzled but kitschy sort of way ... similar to DKC 2's.
So, stylistically, they're *just different* ... and which you prefer depends on how you look at it.