In this Cook vs Howard comparison, you can have both based on current ADP.
Just one vs the other, I would take Howard based on the fact that he has already proven he can be very productive. Worrying about game script seems unnecessary since we already saw what he did on a 3 and 13 team last year. Cook may have more upside due to his receiving abilities but might also give up carries (especially GL carries) to Murray. Also, he is a huge unknown. If the Vikings plan to feature and move around Cook he's going to have to learn (and properly execute) a larger portion of the play book.
Howard is likely to finish as a mid to low RB1 hoping the Cook can come out and exceed that seems like an unnecessary risk to take on a rookie, if given the choice between the two.