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.
The Urban Rivals team is made by lovers of all kind of Collectibles Cards Games and Trading Cards Games like: Magic the Gathering, Dominion, Vampire, Yu-Gi-Oh!, Pokemon, Wakfu TCG, Assassin Creed Recollection, Shadow Era, Kard Kombat and Might & Magic Duel of Champions.