The important point is that shortboards are not fun at all when underpowered. They schlogg through water with the finesse of a barge.
Longer boards with a centerboards are the way to go for highly variable conditions. Compared to my old longboard, modern ones plane well too. Rio M is probably your best bet in the *board line-up. The Fanatic Viper and RRD Longrider are probably good too. If you have more money, the *board Phantom 320 would be great. I personally chose a Kona One, and I'm very happy with my purchase.