rushing, ad infinitum
Well... for me, I always judge the two game mascots on a bit of my own criteria, which includes the general appearance, history, movepool and competitive use. If I feel that it looks awkward or just... not cool, I will probably choose the other. If it doesn't have a nice, attention-grabbing hook correlating with the backstory of the game, I'm probably more tempted to get the other. However, if one of the mascots doesn't tie in with the story they both probably don't so I try not to weight heavily on that aspect of it. Movepool is really important to me, though, because if what I'm going to be battling with throughout the game is sucky, I'm not going to bother. Competitive usage is important to me as well, since I try to do Wi-Fi as much as I can, so if its stats are unmanageable I won't bother with it.
Kyurem-W fits all of my criteria perfectly as opposed to Kyurem-B looking estranged and lopsided and having a shallow(er) movepool.