There's less wasted heat with the pencil plugs. With the loop plugs, a lot of heat goes into the heavy wires that connect them. They glow red on a long glow. Why not put that energy into the combustion chamber, particularly at low temperatures when things get iffy. I'd say that pencil plugs gave me about 10 more degrees of cold starting ability (maybe combined with synthetic oil) on my 77 300d. Not to mention the fact that if you lose one loop plug, you lose them all.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13