That really depends on your preference. Your arms don't really get cold if that is what your concern is. Some prefer the sleeveless suits due to increased mobility at the shoulder but for the most part, most triathletes wear a full sleeve wetsuit. What have you used in the past and what have you trained with?
I'd rent a fullsleeve then. Do make sure you get at least 1 to 2 practice swims in the wetsuit before the event. They do take some getting used to especially since at first they feel quite constricting.