Current USA vs. Coralife...

[Chin_Lee]

if you want to keep anything and everything in the 20, 1x250 is more than enough. Many people have been successful using the general coverage of a 250W to cover a 2x2x2 space. I would even consider the 150 HQI model for a 20g.
For the lighting options, given the tank size and conditions of which you have posted, the Aqualight fixture is really your only choice. The Orbit will be overpowering for many of your corals in a 20g and it will easily overheat unless, like previously mentioned, you have a cooling method in place.
If you were looking to upgrade to larger tank tank in the future, I would consider the Orbit.
My opinion is if you are able to get a bigger tank, you should consider doing it now since the cost of a 20g tank compared to something a little bigger (ie 30-50g) will be a fraction of the cost between the two lighting fixture of which you are considering. its about 600 bucks for the Coralife, a tank upgrade to something bigger will only be $40-50.

[Ruth]

To further what Chin said I would also consider going with a bigger tank to start off.
There is a lot more that good light required to keep healthy coral IMO. Water quality is your first and foremost requirement for keeping healthy fish and corals. I would rate water quality and water movement ahead of light when it comes to the health of a reef system.
With a 20g tank it is a lot harder to keep pristine, consistent water quality than it is with a larger system - it can be done with a low bio load, good skimming, good water movement, meticulous water changes and cleaning.
JMO

[andresont]

I second the opinion that water movement and quality goes beore light.