Both players at their very best I chose Jason Day every time. His short game and putter are untouchable when they are on, but he's streaky.
If the question was more wins over the course of any given year I would go with Dustin Johnson. His length and wedge game week in and week out will net him multiple wins even when he's not firing on all cylinders.
If the question was more wins over the course of any given year I would go with Dustin Johnson. His length and wedge game week in and week out will net him multiple wins even when he's not firing on all cylinders.