The plant light produces light spectra that the chlorophyl in the plants needs to create simple sugars that are the building blocks of plant material. The power glo produces a wider range of spectra that will show off the fishes colours. If you place the power glo in the front of the fixture, the light will reflect off the fish as they swim across the front of the tank, showing off their colours to the max.
If you prefer to max your plant growth, you should use the gro light. It doesnt hurt to change up the lamps once in a while. I do that with my display tanks ju st to get a differant look occaisionaly.