Build something really, really simple in HTML5 and the same thing in JavaFX (using SceneBuilder).
Maybe just doing the layout of the game with no logic at all.
Deploy both of them and try using it.
You'll be in a much better position to decide what is right for you than if somebody offers you up a list of pros and cons, some of which won't really apply to your specific situation. At a minimum create the pro and con list applicable to your app rather than asking somebody else to create it for you.
There is a decent summary of JavaFX features here.
Similar resources that list HTML5 features are easy to find with google.
Then you can compare the feature lists if that's important to you.
I've read a few html vs javafx comparisons on the web and most of them, in my opinion, are pretty misleading, so I wouldn't bother much with them.
The best discussion (by far) I have seen (and one that I don't believe is misleading) is:
JavaFX: The 10,000 foot view (even though it's a year out of date).