If you are okay with both and his headphones are the more expensive ones I'd let him be the final judge. Give him both to trial for a week and have him return the pair he likes less at the end of the week. No hard feelings, and if there are, well it's on him.
The difference in value is not huge so my guess is you'll both come out ahead with a straight swap, as opposed to trying to sell them and then purchase the new ones you want. You lose money every time you do a trade in anyway.
Also consider if one set is significantly older than the other that might also influence things.