Numbers can be determined using stats of previous sales of equivalent copies, but in the end, it really comes down to which book YOU'D rather have. If you'd rather have the Hulk but think it's worth a % less than the Avengers, communicate that to the person you're dealing with and see if there's room for you to get a little cash in the deal. I know what it's like to have a "patch" in my collection ... a placeholder until I find the copy I REALLY want. It might be beneficial for you to consider the marketability of the Avengers once you've upgraded it. Do you think it would be an easy sell with that pq? Good luck with whatever you decide and remember, it's good to have what you want and to want what you have!