AN ACR FLC FLYING B767 ACFT TO EDFF DSNDED PRIOR TO THE LEGAL STEPDOWN FIX FOR THE VOR APCH.

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WE WERE INITIALLY CLRED FOR AN ILS APCH TO RWY 25L. JUST BEFORE VECTOR TO BASE LEG; THIS WAS CHANGED TO A VOR APCH TO RWY 25L. I FOUND MY APCH PLATE AND BRIEFED FOR THE APCH. WE CONFIGURED THE APCH PER COMPANY POLICY WITH MY SIDE IN MAP MODE AND THE FO IN VOR MODE. AS WE TURNED FINAL APCH; I NOTICED A DIFFERENCE IN ALIGNMENT BTWN OUR 2 INSTS; SO I BEGAN TO XREF THE FO'S INSTS WHICH WERE IN RAW VOR MODE. WHILE I WAS DOING THIS; APCH CTL CONFIRMED I WAS 1/2 MI N OF COURSE; BUT WE CORRECTED QUICKLY TO THE COURSE. THE INTL RELIEF OFFICER WAS ALSO MONITORING THE APCH. AT THE 9 MI FIX WE WERE REQUIRED TO LEAVE 4000 FT AND DSND TO 3000 FT. BECAUSE OF THE HIGH WORKLOAD; I SAW MY MILEAGE INDICATOR READ 9 MI (IT WAS READING DISTANCE TO THE OM; NOT THE VOR) AND I BEGAN A DSCNT. I HAD DSNDED 400 FT WHEN I REALIZED I WAS NOT READING THE CORRECT MILEAGE (THE FO'S VOR RAW DATA) AND BEGAN CORRECTING BACK TO 4000 FT. WE WERE THEN GIVEN DSCNT TO 3000 FT BY APCH CTL. THE REST OF THE APCH WAS NORMAL. IN THE FUTURE; I BELIEVE THAT IT WOULD BE WISER TO HAVE BOTH CAPT AND FO SET TO RAW DATA IN THIS CASE; IT IS JUST TOO CONFUSING TO HAVE BOTH SIDES USING DIFFERENT DATA IF FOR SOME REASON THEY DON'T AGREE.