Do you know yet what kind of room you are considering at BLT (i.e. studio, 1BR, 2BR)? If you end up being five with the grandparent included, I'm pretty sure that you would have to shell out for a 2BR since I believe the studios and 1BRs only fit 4.
I would always choose a regular hotel room over a studio (plus the BLT studios are the smallest on property). The 1BR is really nice with the full kitchen, but the kids would still have to sleep on a pull-out sofa and it won't fit 5 if the grandparent joins. How old are your children? That might make a difference in their willingness to sleep on a sofa bed. A 2BR would be awesome if in the budget and would definitely be the choice over a room in the tower if money is not a concern.
I would probably opt for a tower room if you end up being five for cost savings purposes. Plus, it is fun to be in the middle of the magic in the tower and have flexibility if the grandparent does decide to join. Go for the MK view if you can swing it! So awesome to have that castle view at night.
ETA: Oops! Glad to see that I was wrong about the 1BR only fitting 4 people. If the grandparent doesn't mind sleeping on the pullout chair, then that would definitely be a good option. It would just be a matter of deciding whether you care more about the extra space of a 1BR (and full kitchen) versus the option of being in the middle of everything in the tower and having your kids be able to sleep in an actual queen bed/grandparent in a daybed. A 2BR would certainly be more comfortable than the 1BR, though, if it won't break the bank.