I would say 617 for reliability and availability, and 603 for power, ease of modification and weight. Both do not require long-block modifications for power increase, but the head is a power limiter on the 617.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine