GRPs vs. time spent: the great Atlantic divide

At a recent IAB UK video event I asked the audience whether it was more important to plan online video campaigns to optimise Gross Rating Points or to get consumers to spend more time with the brand. 

What was the answer and why would it be different in the US?

Over 80% said time, or attention, was more important, yet at a US equivalent I’d expect exactly the reverse. So, who’s right?