My experience is that when people say powerhead, they mean a pump with one inlet and one outlet. (Think inlet pipe and outlet pipe.) A circulation pump is kind of like a fan underwater. The fan part is surrounded by a cage of some sort to help keep critters out.
The Maxi Jet Pro line by Marineland is convertible between a powerhead and a circulation pump. I have used these, and they are a good value for the money, BUT the mounting system stinks. If you use the clip version, it might be too close to the water surface to keep from sucking in air and making a lot of noise. If you use the suction cup mount, the suction cups tend to not hold very well for more than a day or two, and the pump falls a creates a sandstorm in the tank.
If you're looking for a circulation pump, I hihgly recommend the Hydor Korallias as a good value. They use a magnet and suction cup mount that holds very well and are pretty quiet. The down side is they are kind of big.
There are several other brands out there as well, some of which have a boat load of features, but I don't have any direct experience with them.
I will say i have used Aqua Clear powerheads, and they are workhorses and last a long time, but tend to be a little loud IMO