In general longer is a better stimulus. The reason that duration is such a powerful stimulus to aerobic adaptation is that it is the gylcogen depletion in the working muscles that starts the cascade of the signaling pathway.
I say in general because it matters where you are in the your development. If you are training for say a vertical K race or marathon where the intensity is higher than during a month long climbing expedition at over 6000 meters, you will need to do some shorter and faster training sessions. That is because the endurance limit of the fastest twitch fibers used in your event will be what limits your performance in events under 2-3 hours or events that have a great deal of elevation gain/loss where pace changes dramatically.
See the Frontier Fiber Model discussed in the new book Training for the Uphill Athlete
Long slow aerobic work is necessary to develop this base. But that low speed work focuses the adaptations in the ST fiber pool. You will probably need some FT fiber adaptation as well for most events. So, the mix of longer slower workouts with shorter faster workouts after the aerobic base is well established.
Scott