Planes Material works creating a mental ray material that uses Forest Texture + Forest Shadow, so if you don't include other shaders (as Photon or Volume), the result will be the same. We suggest you always use Planes Material.
XShadow doesn't work with Mental Ray. Max use automatically Raytraced Shadows when XShadow is detected.
However, Forest 2.4 is not the best option to work with mrSun/sky, because it doesn't include the improvements done for Mental Ray of Forest 3. We have rewritten the shader to provide better compatibility with HDR solutions (as mrSun). I strongly recommend that you upgrade to Forest 3 if you can.