Wait for the new cards to pop in the NB pack and spend them for 15 packs ... get all the new cards in the shop and all bad send to me for the good advice
In 2-3 words : NB packs when ther are new cards
Personally, I've been purchasing few times and getting free offers with the iphone apps, I recommend you to buy extended all at once. NB is good too if you have enough to open 3-5 packs at the same time and also if you have enough collections already NB is really damn good. hehe
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.