The 2.6 I believe had MB Tex instead of leather, and a couple of other subtle differences. The engines are both M103 with different displacements and are both bulletproof. With that said, they also share the same common issues such as OVP, front U-seal, etc. The 2.6 is much less powerful and I don't think it is much more fuel economical.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280