Why does everyone recommend saving all my summons for Sun Wukong when as a new player wouldn't saving for the 1+1 banner on Christmas be better?

Basically the title, I've got 117 rare stones and 7.5k gems so I'm ready for either, but it seems like everyone is sure Sun Wukong is going to be the best whatever he is. What if he kinda sucks, do we know his stats and abilities yet? Wouldn't I be better served doing the 1+1 legendary banner or am I confused and there is no 1+1 legendary banner?