A PGDip 120 credits, 60 credits short of a Master of Art at 180 credits. That difference would typically be a dissertation, a large independent project, a large body of work or some other single 60 credit project. So, a PGDip and MA usually require you do complete the same amount of coursework and course work assessment, but the PGDip will stop at the end of the coursework, and the MA will progress to the final project. Both will be at the same Qualification Framework level (SCQF11 in Scotland, for example).
If you're unsure, contact the institutions in question and ask them!
Good luck.
C.miller