I'm assuming this is numbers 26 to 38, i.e the first part of the Pentecost cycle. I have about a third of this on the Cybele CD's and after repeated listening, Delvallee wins over Sandro Muller.
Although the Muller interperetation is interestingly different, the organs he uses are better for the quieter pieces - indeed, the Cybele CD's seem to be recorded quite softly. Delvalle's registration seems to bring out those "hidden" themes, and, to me, Orgue Mystique is so suited to French organs.
As to the content, there are some 60 pieces here, some are fairly ordinary, others outstanding, all interesting. The CD packaging and presentation is good, and there is also a reference table of the chants used for each piece.
But then, why not just go the whole way and get the new 12 CD box set for £60 or so? (To my knowledge this is the only recording of the whole cycle of 250 odd pieces.)