It depends on the types of coral you plan to keep. 10 to 20 times the water volume per hour is a good safe starting point.
The hydor koralias are not bad powerheads if you are on a budget. The drawbacks when compared to a vortech are:
-Less flow in comparison to the size
-Much less random flow as they are constant on
-More pump in your display tank vs. a vortech which has 60% of the assembly outside the tank
-More heat added to the system since the motor is inside the tank. The vortech adds no heat as there is no motor inside the tank. This is not so much a problem this time of year, but it can be a problem in the summer time, when most folks' houses are a little warmer. You are already adding heat via a return pump, a skimmer, a reactor (if you run 1), lighting, etc. The less heat you put in the tank, the less likely you are to need a chiller which is expensive, can be noisy, and also costs quite a bit in electricity to run.
EDIT: Ninja'd by convict!