I admit to limited exposure to both Tiles and Sitemesh. In the last project I was involved with, we used Sitemesh. The project layouts were simple so we had a chance to create proof of concept codes for both, then eventually settled for sitemesh. For reference, we used the on-line documentation of OpenSymphony and a book owned by one of our colleagues titled Professional
JSP Site Design published 2001.
For Tiles to be used, it should be integrated at the start of development. Sitemesh is based on filters so it can be integrated at a later stage of the project. This is good because, you can immediately focus on doing the business logic of the app and worry about doing other aspects later e.g. page decoration/layout etc..
Maybe others could also tell us their assessment in evaluating the two.