If you deem yourself very very Lucky, 2x 20 credit packs might be better. A pack has no doubles, meaning you could receive 2 Ravens by purchasing 2x 20 cred NB packs. But you can get only 1 Raven from opening the 40 cred NB pack.
But like Battery and Bliss have pointed out 40 cred pack is a safer/better choice.
If you can wait, buy the 40 cred NB on next Friday after the new cards (possibly new clan also) are released.