I love 30 Days. I really do. And even though, I don't agree with everyone saying it's bad or it's one of their worst singles (as they said it wouldn't even chart in the top 10, or that it wouldn't outsell MHTO, etc), I think it's too weak for a first single to crack America.
Higher and All Fired Up are massive tune. So are Ego and Notorious. There are plenty of hits they can release, but as flapjack said, I'm pretty sure they are going to release a total new song. It will be released in the US first and then in the UK, I reckon.