Benny will be 28 in a couple months. I agree that it's more likely than not that Benn outpoints Ehlers next year, but it's not a slam dunk. I think there are several universes where Ehlers overtakes him marginally in 2017-18.
As for the following three or four years, prime-aged 21-24 year old Ehlers vs 30-33 year old Benn, I would want the younger player there. If for nothing else the trade value that would massively swing for Ehlers - especially in my league. And the likelihood of more points.