If you are going between to studios than for me you need to ask about room lay out. Since you have a boy and a girl, both with give you a sofa pull out, BRV will have a full, Poly will have a queen, both will have a pull down Murphy twin bed. And both will have a queeen bed. Here is the big difference, the poly bath room will give you a shower and toilet in one, and a tub with shower and sink in the other, so mom and daughter can shower in one while dad and son use the other in morning and you all get done faster. At BRV you have only a tub with shower and toilet and the sinks are outside the bathroom.
Next thing to consider is are you going to use the pools a lot. My favorite would be BRV, but your kids favorite is going to be Poly. The difference and someone else can chime in here, is at BRV everyone can use both pools I am told, but at poly the quiet pool is just for the villa people, although again your kids won't want the quiet pool, my DGD is 12 and she loves poly pool the best.
As for resturaunts I say hands down BRV for quick service and a tie for table service. I do not like the quick service at poly, don't like the food, don't like the lay out and is always way to crowded and packed. But that could just be me, others will say roaring forks is small without options. I prefer whispering canyon over Ohana but they are both good.
One has the monorail, one has a boat ride, both are fun, one is slightly faster, but I find the monorail also much more crowded.
For your kids your best choice may be poly, it would be for our family. Yet our families favorite is BRV and we do a split stay many times so we can enjoy both.