Quite a big difference. Thomas has signed thousands of items, not to mention the usual fraud factor in sports memorabilia. Cullen rarely appears in public.
When it comes to off the wall items, the truth is they aren't worth much more than had the guy signed a ball, photo, or whatever. For baseball, signed bats are worth a lot more, as are jerseys. That's only because they CHARGE a lot more to have them signed! Sports-wise, personal items of the player (Game used, or other personal stuff) would be worth much more. Those are unique to begin with.
The OP may find someone who is a Saturn fanatic who would pay more, you never know. I've met several celebrities over the years who have been licensed onto games, but I rarely think hmm, I'd like to get them to sign a game. I usually figure it's better to have them sign an item more closely related to their work. Diecast car, baseball, photo, fake prop, etc.