I would go with the Poly. We (DW, DS8, DS6) have stayed at both the Poly and YC and while SAB is great, it may be too big to keep track of your kids while swimming. The volcano pool at the Poly is a good size, has zero entry for the little ones and is located adjacent to the beach. It is very relaxing. We also loved the monorail. The buses at WDW STINK!!! I hate them. The convenience of taking the monorail to MK and Epcot far out ways the BC/YC advantages. Also, we liked the atmosphere at the Poly. It is much more relaxing to us. You do not have to trek through the big lobby in your bathing suit to get to the pool.
As for concierge, we have found that it is worth it at YC and the Poly, but not at CR Tower Club (where it is limited and less expensive). We ate every breakfast there, plus midday snacks and drinks. We would eat bigger lunches at the Parks and then have a light dinner at the concierge. It was great to take the food down to the pool at the Poly and eat while the kids swam. They also have great desserts. If you are willing to adjust to this, it is worth it. If you want to eat at a restaurant every night, then it is not. Jay