Right now you can get a room at the Royal Pacific for one night (either the 21st or the 22nd) for $289+taxes (comes out to about $320 all in.)
If you want to have the best chance of riding everything you want in IOA (and possibly your favorite rides multiple times) then an EP is your best option. My guess is IOA won't be at its most crowded nor will it be completely empty. By getting the room you'll save $40, but you will also have the option of early entry (if IOA is the Early park for either day.)
So my suggestion is get the room, arrive early, park at the hotel, water taxi over and hit the park until you are tired. If there is any chance you can make a 2nd day happen, you would now have EP for that 2nd day for no extra charge.