As far as I can see the rewards are the same? does the AI play differently, are the cards rigged?
On the increased difficulty, the NPCs change their playstyles slightly and act less random. It makes it a little easier to try to strategize and makes the game feel a bit more like a "real" poker game, but it's still difficult to properly bluff, for example. Reviews say that the poker experience for the player is more consistent with the higher difficulty setting.
Basically, if you are an experienced poker and Texas Hold'Em player, you won't find the game very challenging (and may indeed find it more frustrating if you try to apply normal poker strategy to the game), so setting the difficulty higher will probably be more rewarding for you.