Buy Express. Get in line for Hagrid's early (since it doesn't take Express) and then do the rest of the rides in the HP lands. From there, pick a park and finish up all your rides. If you are pushed for time, Express, while expensive, will get it done for you.
Otherwise, start with Hagrid's still and work your way around to what you want as time permits. it's probably a full day in both sections with a stop for lunch and some shopping and seeing the details in the lands. If this is your first visit, spend a good amount of time since there's lots to see.