If you were to choose the Dream cruise, and ultimately not be able to dock at
Castaway Cay due to weather, would you regret your choice? I'm not experienced enough to know when this is most likely to be an issue, but it does happen from time to time. I just wouldn't want you to choose solely based on that stop and then end up hugely disappointed if it didn't happen.
I'm usually in the 'longer is better' camp and I personally prefer the 'classic' ships. That said, if it's your first cruise, and you're planning on being in an inside stateroom, I might say do the Dream instead just because you don't know how you and your family will like cruising, and 5 nights is a good length to get a good taste of it but not feel trapped if you don't end up loving it. I feel like I've also read threads about Galveston having really rough waters (I have not sailed out of Galveston myself), so if you don't know how your family will do seasickness-wise, that might be something to consider as well.
I think you could have a great time either way though!