The former is recommended for existing Tangram users of 2.08 and below to switch to, to trial new Tangram features such as Tangram::IDBIF or outer join support. The latter is recommended for people starting new Tangram projects, and is mostly a namespace re-organisation and splitting out packages that were combined in single files into a "proper" structure.
Tangram 2.09 is more clear about when the test suite is disabled, which should lead to fewer false negatives on the CPAN testers reports (apparently people think it's a good idea to lodge a test failure report after answering a question "Do you want to run the regression tests?" with No).
Other than that, there have been various fixes and improvements. Users switching from 2.07 and below will notice a much improved TANGRAM_TRACE output and many new features as mentioned in the ChangeLog.