Why SiteMesh instead of Tiles?

1. SiteMesh is much easier to configure and use (less typing and easier to understand)
2. Using Tiles, you need to have your forwards go to a "tiles page" versus the direct JSP. SiteMesh takes the approach that your page (your JSP) doesn't even know or care that it's being decorated.
3. Using Tiles, each individual page you want to go to has to be associated with a layout - Major pain! Every time you create a new JSP that you want to forward to, you have to create another tiles definition and associate it with a layout and forward to the Tile page (versus the JSP). With SiteMesh you can simply set up a URL pattern and all your pages are decorated with the layout you choose.

On the other hand there is downside of using SiteMesh. SiteMesh stores the entire content of your HTML body into memory before it decorates it. If you have some very large pages, such as might happen in a reporting application where you don't have pagination implemented and end up with one large page of rows, you could end up with severe memory problems.
You can read this http://today.java.net/pub/a/today/2004/03/11/sitemesh.html post by Will Iverson about how to use SiteMesh.