Having owned both engines, I would definitely go with the M104 (IF the wiring harness and head gasket problems have been properly taken care for!).

Much smoother, more economical, better torque, and, believe it or not, easier regular maintenance.

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A. Rosich
CL 500, 1998
S 500 L, 1998
E 320 T, 1995 [Sadly sold ]