Let's See:
GAMEPLAY
Days
-Recycled worlds we have been playing since the Big Bang. ******* AGRABAH!!!
-Tedious and often repetetive missions
+ More difficult than Kingdom Hearts II
+ Panel System (this is debatable)
-Does very little to distinguish itself from its predecessors aside from Limit Breaks.
Birth By Sleep
+ All worlds are essentially new, even Radiant Garden and Olympus Coliseum.
+Even while traversing the same worlds through three seperate scenarios-it was always new and refreshing due to some areas being exclusively accessible to either of the 3.
+The command decks, command styles, shotlocks, and plethora of new magic/physical commands borrowed from Chain of Memories make it the most superior gameplay wise out of any KH game to date.
+You can actually die in Proud Moud.
From a story perspective, I nod towards Birth By Sleep. Granted, I
seriously enjoyed Days' story (loved the Trio's relationship)and its ending is by far my favorite in KH history thus far-its pacing and lack of interaction with characters who are not Xion or Axel sullies the immersion. In fact, there is no genuine sense of "conflict" in the story aside from Sai'x being a complete jackass earlier on and by the time **** gets real...it's treading dangerously close to the end of the game. Sai'x was satisfactory, but the rest of the Org. XIII was underwhelming-they could have been far more involved. Xemnas makes like, what, three appearances tops?
Still, I must give its ending credit for actually making me sad in a KH game, something not even Birth By Sleep accomplished. Aside from that, Birth By Sleep wins in every category (and BBS' ending was still
awesome) but the "faint traces of hope" made it less emotional for me. Alas, no character in Days can compete with Vanitas so BBS wins by default for having superior characters
I do not even see a reason to justify my preference when it comes to the story.