Depends. Do you find the length of Goodfella perfectly satisfying, or would you enjoy an additional inch of insertable length? That's the difference. They seem to be molded exactly the same, with exactly the same details, texture, feel and girth, but because of where the balls are placed on Goodfella, it's about an inch shorter than Mustang.
That's why, in spite of how much we love the balls on Goodfella, we're listing it on Eden classifieds and keeping Mustang. We keep hoping Vixen will finally listen to us and make a larger version of Goodfella, with the same wonderful 360° balls that sit outside the harness, but with AT LEAST as much girth and insertable length as Mustang (preferably it would be the size of Johnny).