Learning on a shortboard

In the fall 2008, im going to UCSB (university of california santa barbara) i've surfed 4 times before and i can stand up and ride a wave about 50 percent of the time, should i go ahead and just buy a used shortboard im thinking 6'8'' at play it again sports then buying a new shortboard when i get better? Or should i get a soft top 7 foot? Im 5'10'' 155 pounds.