You can sign up for either. Since this is the Half Ironman group, neither distance should be too daunting. Just use it for training and race specific practice (open water swim, setting up transition, etc). The schedule calls for a 3-3.5 long ride on Saturday so come down and enjoy company for this longer ride.
Most people arrive Friday so they can join the group workout early Saturday morning. The clinics then start around 10:30/11 am and run through the late afternoon. You can drive down early Saturday morning if you are a (very early) morning person. I would recommend reserving your camping spot now. It does fill up and many people were not able to secure a site last year if they waited until the last minute.
Look forward to seeing you there!