I used to own and play a Rolling Glide Iron Cobra double pedal. This model has a concentric cam, which means, the cam (the sprocket where the chain travels) has a circular profile. If you look at the pedal on its side view, you'll notice that the cam is round and it is mounted on its center. Because of that, the pedal offers a uniform response. Based on my experiences, I enjoyed the smooth, even response of the Rolling Glide model.
Meanwhile, the Power Glide model's cam is mounted off-set, meaning, it's cam is mounted off-center. Because of the off-set mounting, the distance travelled by the beater/chain/footboard mechanism becomes shorter, and there's more speed and power compared to a concentric cam pedal. So if you're a drummer who needs to take advantage of quicker and more powerful pedal footwork, the Power Glide model can become very helpful. I've tried a Power Glide model, and for me it feels easier nailing down those faster bass drum licks.
Both Power Glide and Rolling Glide (and the Flexi Glide) offer virtually the same features. The differences among these three models are its cam systems and drive systems (chain or belt).