I would think Hard Rock would be the way to go.Gringots may not have express yet.But I would think onsite premium hotels will get preference over cabana bay for getting into areas.With opening They will be restricting access to Hogsmead,Diagon Alley and to re-ride the Hogwarts express.
Riding the Hogwarts Express will take you outside the entrances to both.
I think the exit to IOA side will be near Sinbad and a wall/barrier will be in place to make you go down by Mystic Coin place.Before turning back to Hogsmead (to control the number of people in Hogsmead)
The Exit for the London side will be close to the San Fran area.
My point is that even if you get in early to Diagon and Gringots .You will have to exit that area to ride the Hogwarts express.
True, you have a point about the Hogwarts Express. Once you leave on the train, it could be more difficult to get back in if you're not a hotel guest. If there's hours long lines just to get into the areas, I can't believe they would make you get back in that line because you got on the Hogwarts Express. I wonder how they are going to handle that, with so many entrance and exit points, unlike the Hogsmeade opening in 2010.