I wouldn't be surprised if the 35i is slightly cleaner than the 35d, as the 35i is new technology and the relevant 35d drivetrain debuted back in 2006 in the 535d (the M57 itself came out in 1998, the one sold in the X5 debuted in 9/06).

For comparison purposes, the xDrive40d is cleaner than the xDrive35i with lower fuel consumption and CO² emissions.