I would go to both parks to see the Harry Potter stuff, it is definitely worth it and take Hogwarts express to go between parks. I really liked the Gringots ride. Forbidden Journey is fun too, but I have not felt so great a few times after riding it. There is the new Hagrid coaster that is suppose to be really good too. Staying on property does get you FOTL, but not for Harry Potter. Staying on property will get you into the Harry Potter lands an hour earlier though, so keep that in mind, no FOTL for HP.