There are differences regarding other applications that WinRunner can interface with such as Data Junction, that XRunner does not interface with. Commands for Data Junction start with db_. XRunner's TSL command reference guide does not list these commands. Nor are commands listed for ODBC or for data driven commands which begin ddt_. Test Director, as far as I know, is Windows based and does not run on UNIX so the tl_ commands for connectivity to Test Director are also not present. As coling says, GUI mapping is also a bit different with GUI maps between XRunner and WinRunner definitely NOT interchangeable.
Other than these exceptions, the TSL is exactly the same.