If you're a huge HP fan, and only have one day to visit, you probably want the one day / two park ticket. Plan on Diagon Alley in the AM and do Gringott's as quickly as possible ... then take your time to explore the land and knockturn alley. I would do a late breakfast/early lunch at Leaky Cauldron, and then take Hogwart's Express to Hogsmeade. You can usually get on the Forbidden Journey single ride line rather quickly, and then you can go back for a castle walkthrough tour if you want to see the castle (which I recommend). Hogsmeade will seem a little small and quaint after Diagon Alley, so I would probably split your time about 70% Diagon Alley and 30% Hogsmeade.
Don't know when you plan to go, but they are running some great Universal Resort deals now - just saw an email for Portafino Bay for something like $195 per night. If you stay the night prior at a Universal resort, you almost always get an early admission to one of the HP sides, as well as front of line access to the secondary rides. It could be a good deal for you if you're trying to maximize what you see in one day (plus I love the Universal Deluxe Resorts, especially Portafino Bay)
Have fun ! I'm only a mediocre HP fan and I thought Diagon Alley was the best thing I saw in a our 2 week trip last summer. It was fantastic. My grumpy old butt even loved taking my 10 year olds wand and casting spells throughout Knockturn Alley - Awesome !