I'm exactly in the same place. We are a family of 5 with young kids, we have LOVED renting POLY it is by far our favorite resort but we have to consider BLT as an option for when the kids get older and we need more space. Decisions Decisions!
Remember, the studios are Poly are much bigger than the studios at BLT..
Poly occupancy for studios is 5
BLT studios occupancy is only 4..
So at BLT, you would need a 1 bedroom for your family. At Poly, you'd still have the option of a studio.
And when you really need extra space, 2 studios is typically "cheaper" than a 1 or 2 bedroom.
So in many ways, the "space" argument favors Poly over BLT.