Both. Shortboard when the waves are good and longboard when they aren't. I just bought a longboard and have been having a blast on it. On days I used to consider unsurfable I can just grab the log and go get some good rides. These past few days I've had some super fun sessions on it when I would have never considered paddling out on a shortboard.
Learn how to ride both well which does take time. Ride both long and short on the same swell expand all horizons. Learn how to walk the longboard no chicken hopping. Cutback, trim hang five and ten. Rip with style on both you will become a much better surfer. Thats just for starts. P.S Learn the history. Surfs Up!
Whaaa? Fewer waves? If you are catching fewer waves then your not doing it right. Ankle to knee dribblers or just in the mood then Longboard. A peeling wave over knee high on the right shorty, I can catch anything a longboard can.. sure maybe I'm not cruising but I'll fit more maneuvers into my ride than he will. Small wave= small board