This is a really good poll.
I think they're roughly equal as playmakers. Turgeon is clearly the better goal-scorer. Backstrom is far better defensively.
Both were roughly even in the playoffs - meeting expectations, but not exceeding them.
The biggest difference is Turgeon was the top forward on his team for much of his career. He had some huge years (leading the 1993 Islanders in scoring by 45 points, and he led several other teams in scoring by 15+ points - 1989 and 1990 Sabres, 1994 Islanders). Backstrom, of course, has spent most of his career supporting Ovechkin.
I really value Backstrom's defensive play, but if I had to draft one of them with the expectation that they'd be a #1 centre, Turgeon's offensive talent is too good to ignore (so I voted for him in this poll). On the other hand, Backstrom has been the perfect complement to arguably the greatest goal-scorer in NHL history, and I can't envision Turgeon filling that role nearly as effectively.