The WHATWG spec is the place to look for what’s new and evolving. The W3C spec, once it goes into Last Call, is the place to look for the official milestone that is HTML5. In practice, the two specs will be pretty much identical for quite a while yet. But the truth is that authors shouldn’t be looking at specs to decide what to use—look at what browsers support in order to decide if you should use a particular feature—look at the spec to understand how to use features of HTML5.