Both part of the NWOBHM. Motorhead started in 1975 & Venom started in 1979. Both mentioned as major influences for thrash metal etc. Both got considerable respect from both metal fans & Punk fans.
Venom quotes Lemmy as a smart man & great axeman. Lemmy quotes Venom as not technically skilled musicians saying they often 'fake it' but later ammended it saying they are "all right". Venom quote Motorhead as big influence. Lemmy has showed little respect to Venom.
When interviewer Sam Dunn tells Lemmy that he himself was "blown away" by Venom back when he was twelve, Lemmy responds, "Anything will blow you away when you're twelve. A day at the beach will blow you away when you are twelve."
Cronos:
"'head have always had a great response in Newcastle, Geordies really relate to them, Lemmy is a smart bloke and a wicked axeman, true rock n' rollers".
Until recently Motorhead would have always won this poll hands down but with the more recent rise of black metal groups quoting Venom as their greatest influence, I'm sure Venom may have equalled some of the balance.
For me its definetely Motorhead. I bought Venom's Welcome to hell & black metal back in the 80's and I liked them but nowhere near as much as Motorhead's Ace of Spades, Iron Fist or Orgasmatron. That's without the great newer material they keep producing. I could never really take Venom serious and could never dig their sense of image. They would make me laugh. Motorhead on the other hand with all their masses of hardened looking Hell's Angels & other biker gangs following them was to me just plain cool

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My fav'
YouTube - Motorhead Orgasmatron