The Strawhats didn't need to go to the New World immediately. Did you guys just randomly forget how many battles the SH went through. None of the other SuperNova even compare to be honest. The Strawhats have defeated Buggy, Morgan, Alvida, Kuro, baroque works, Mr. 1/2, Crocodile, Bellamy, Enel, Cp9, Moria and his crew, impel down for God sake and the war. I promise you Law or Kidd hadn't defeated or even faced 4 Shichibukai in battle. The Strawhats also saw the power of an Admiral(aokiji) way before Kizaru. They already saw the strongest power there is. Especially Luffy and Zoro. Luffy fought alongside the likes of Whitebeard, Jinbei, Crocodile and Ivankov against the worlds strongest while Law and Kidd were pissing off.
The Strawhats didn't need anymore battle experience, they needed training. They would destroy Law or Kidd's crew easily. I've been saying this for the longest. Law and Kidd aren't even as strong as Zoro and we've yet to see Luffy's true power.