Is quantity more important or quality? This question comes up in many facets of life.
Is hard work better or smart work? Is trying multiple things more important or carefully selecting the few you try?
The fact is that we much more control over one than the other.
We can easily measure how long we work, while its kinda squeasy to determine how smart. Of course, if your task is very simple with one metric - then how much you moved that metric is what matters. If you are a worker on assembly line, only thing which matters is how many cars you fitted.
But modern jobs are hardly so simple.
Still, the only thing we really can measure is how long we worked on something. How smart was it is only validated later - when you measure how successful that task was. At least working longer hours lets you try more number of things - and hopefully you stumble upon something smart.
So, there is really no dichotomy here. Work as hard as you can - at least in early days when you don't have much idea of what smart means.