Its largely a question of maneuverability vs. hull speed and extra storage. If you want to do big days or touring, then the hassle of a longer boat (namely car topping and storage) will likely be worth it.
That said, my stepfather has an 13, and its remarkably capable for a long day out, and quite fast (with turbo fins). Impressive boat!
"Never mistake a clear view for a short journey." - Cowboy Proverb
Graduating from adventure junkie to dealer. Get your fix at http://www.fasterfarther.com