I'm about to have a go at creating an App, should I go with the facebook, or wait a few days and go with the new myspace API?
Facebook. It's very good and quick to get started with. I can't compare to myspace though as I can't stand that site. It's awful.
Or go with both? That said, given the two attract very different types of people why surely you should decide on the basis of which most closely fits your application type.