I didn't actually like Jack of the short story. *waits for lightning bolt* And I always thought that Ennis was used a little by Story!Jack. There was Ennis opening his heart out to Jack when Jack had been diddling around, then lying about it. Yeah, I know you could say he did it to spare Ennis's feelings but putting his whole character together, it just comes across as saving his own skin and getting himself out of a tricky situation.
One big place where, in my mind, Story!Jack and Movie!Jack don't reconcile very well is the scene with Jack's father, where he tells Ennis that Jack had another guy coming up to help. I can well believe that of S!J, but M!J just seems a lot less likely to have taken that course of action.
But then, of course, these two guys do not actually exist - they were created. Much as we'd like to think otherwise, giving two characters a personality is a lot different to two real people living in the world.