Uber would most likely be the best and cheapest.
You can buy online and print out tickets to save a few bucks. Otherwise, just buy at gate since no real discount for a one day pass. It may be busy, with HHN in full swing. I would get there early. Check the website, I think early entry for onsite guests is IOA, so start in the Studios when they open. Universal has the express pass. It is date and park specific, with no refunds. So, I would get there, see how crowds are, and purchase in the park if you think it is worth it. It is for sale all over.
If you are into Potter, make sure you get a park to park. Only way to ride the cool Potter train between parks.
Have a great time, and if you think you would return, look into upgrading to an Annual Pass while you are inside the park.