At times I think we believe you can't actually teach players anything. You can't teach size, speed, hockey sense, vision, etc. There's got to be something you can teach hockey players

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You can't turn someone into a speed demon, but you can improve people's skating which should give them more mobility. Not to mention every guy doesn't have to be a speed demon to be a very dangerous player. Good players will find a way to make use of their best assets and minimize their weak points.
I think both Marner and Strome are really good players and teams will end up pretty well off with whichever. I don't think either is a hugely safer pick.