Polygon has the lowest transaction fees. That's a huge advantage in DEFIs and DAPPs. BSC transaction are nearing $1 while Polygon transaction are around 1 cents.
BSC on the other hand has a support of largest centralized exchange and it has many popular and high volume projects like Pancakeswap. Though polygon is rising quicker, there are no prominent and large volume defis for Polygon like Pancakeswap. But with similar working, it's likely to get some good projects sooner. I believe the lower fees of Polygon is going to make it better than BSC chain.