As another recent newbie, I'd say that if you have the time, money, and inclination, definitely sign up for both days. By the time you drive to the track, set up your pits, and get lodging (if required), you may as well maximize your time on the track and do the whole weekend.
You'll likely get a little over two hours of total riding time per day, so that's somewhere in the range of four-five hours total riding time for both days.