We have both a Costco and a BJ's membership, but I prefer BJ's. IDK about your area, but both of the Costco near me are in areas that are poorly planned, traffic wise making a higher volume of traffic even worse than it really should be. I'm beginning to think they need to hire new engineers to plan their locations (ever single Costco I've ever visited has been a hot mess, traffic and parking wise).
The biggest advantage that BJ's has, in my eyes, is discounted Disney gift cards. They routinely are 4% off (I think) but if you watch your emails they run additional discounts, either % wise or $ off certain gift cards, Disney often being one. Most of the gift card discounts are online only, so don't expect the store to have the same special or even match it, they will not. If you do buy Disney gift cards, choose one with a set value and not the one where you choose the value- those are never discounted.