They should be completely compatible. And the extension doesn't mean anything, it's the internal format of the file that makes a difference. The extension is just a cheap way Microsoft came up with for creating program associations, rather than do a proper job of it and use metadata embedded into the file.

Office 2008 does lack support for VBA, which may be causing problems. In which case, you'd want to try rolling back to Office 2004 which still has VBA.

That should really be the only difference between the two aside from how they look. They should otherwise have feature parity. And technically Office 2008 has AppleScript support which provides much the same purpose as VBA, it's just only supported on the Mac side.

Finally, Mac is not an acronym, it's short for Macintosh... Like the type of apple... Get it? So it doesn't need to be in all caps.