If all you want to do is ride the train and not actually jump between the parks then no. We get park to park because we like being able to go between the two harry potter worlds in a day to get a full potter experience.
Imo the train is nothing special. You could probably skip it and get the same thrill watching a youtube video on it. But that's just my opinion....it's an ok ride if you do plan on hopping between parks but idk that it's worth the cost difference just to experience it live.
The train is cool and a must for HP fans. The ability to hop between the parks is the real benefit though! The parks are right there next to each other. We treated it like one big park with 2 separate ticket checks. I would hate to be locked into one park/ day.
The Hogwarts Express is a fun ride....Potter fan or not........
As said, ride it both directions too....it's different each way.......handy for getting from one park to another and to get the Potter experience too......
If you can do it with a park to park ticket, I would.