Hi! Welcome! Ask anything and as often as needed. I’m a self proclaimed “Universal Addict!”
I love those parks so much and all their events, like HHN!
In this scenerio, if Hagrid’s is NOT your #1 “must-do” ride then start at USF.
I’ve ridden Hagrid’s, literally, over 50 times. At least half of those are RD rides.
Unless you are certain you will be at the park entrance at least 45 minutes before EPA starts, preferably 50 - 60 minutes, getting on Hagrid’s will take the full EPA time - maybe longer.
Even when I get there an hour early, then I truly RUN/JOG to the ride at the front of the herd, I have never boarded the ride in less than 20 - 25 minutes at RD. I’m usually in that queue, even at the front of the herd for 30 minutes. By the time I get off the ride, the queue, typically, stretches all the way back to Lost Continent / VC with a posted 2 hour wait. (90% of all people at IOA RD are going to Hagrid’s)
However, going to Hagrid’s at RD does help ensure you get to ride it that day. While it operates pretty consistently these days, it can go offline during the day routinely.
IMHO - If waking up 90 minutes early that morning to wait outside the entrance for 50+ minutes and then running to the ride doesn’t sound like fun - go to USF! 
You can see Diagon Alley before it gets too packed, see some entertainment after 10am, ride most things at USF and be back over at IOA by mid-afternoon. There’s only about 8 - 10 attractions currently operating at that park. They’ve closed Rip Ride Rock’it and Fast & Furious now.
There’s a couple of different ways to decide. It will really depend on what are you prioritizes, size of group and if you have any small children.
I hope this helps!