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.