Apart from oxygenating the tank water, air stones are advantageous when it comes to providing excellent water circulation. They help boost water circulation more than an undispersed air feed into the aquarium from an air pump. The bubbles will also bolster the surface area of the water more...
I used to position powerhead on one side of the back of the aquarium. Water flow will be limited to one side of the tank. But you have two so I think you could put them on both sides of the tank
I heard that SeaChem filter is a great filter for 55-gallon fish tank. Two major wins are the surface skimming functionality and the submersed pump location. With the pump inside the tank, under the waterline, the pump is self-priming or more accurately, doesn't need priming.