Ace is a static page generator like Nanoc, Jekyll or Webby. How is it different? Well, it's better :) ! If you need to generate a lot of pages on the fly, Nanoc is a hassle. It provides helpers, but helpers are – let's face it – programming style we used to use years back when we yet believed that PHP is actually a really nice language.
Also, last but not least, Ace has a real template inheritance. Layouts are for kids, real men use template inheritance! What's the advantage? It's incredibly flexible.