Well, from the details, I'm hoping it's a sequel (guess wikipedia will just have to update accordingly, simply because I don't feel like shifting through dozens of articles on it, especially since only the teaser's out). I won't be as excited for it if it's a prequel, but that's just my personality. John Marston is an interesting character to be sure, but not so much that I would want to live his past, rather than a new adventure about Jack and his role in the West.
The username's WanderingJester, but I'm pretty sure for some reason in order to find me you'll have to look up willinhood. Try both and see which works