I've probably typed something like this before but...(keep in mind my perspective is from a large Hub, and not preload)
Most of the responses you are going to get are from union employees who have grown to hate management so much that they will try to dissuade you from taking a job as a p/t sup. I've always said, it depends on your goals what you should do. You say you don't want to stay at UPS after you finish school. If you are 100% certain, then I would recommend becoming a p/t sup. You will learn a lot, in particular how to deal with every type of employee. If you care about your job, it will be both stressful and fulfilling. The pay for a low seniority UPS employee is better than the union. You will leave less physically tired, but more mentally exhausted from the stress (and I really disagree with hoaxster, the pt sup gets all the stress and pressure, and the ft sup blames everything on pt sup).
Having said all of that, be careful with today's job market. A degree isn't a guarantee for a decent paying job anymore. You don't want to be stuck as a p/t sup without a job. At least by staying in the union, you will eventually have a guarantee of a great paying job, as long as you don't do anything ridiculous to jeopardize it. By staying in the union, you also only have to worry about the work you do, not what the 10-20 employees that work for you do.
Neither job is easy, but really think about the future before you make a decision. Management worked out for me, but I know plenty, even with FT sup jobs that regret their decision.