A 20 long is only 12" deep. Is that what you have?
Then you don't need crazy powerful, not that the AI is really any more powerful than a Mars Aqua. And yes, with the Mars Aqua you do give up some of those features that humans like (but corals couldn't care less about). And 2 would cost less than $200.
If you like having a lot of the bells and whistles (personally I do) consider a 24" Reef Breeders Photon V2+ for $400. It has 6 channels of color control, Cree and Osram 5w and 3w leds, super slimline aluminum case, wide spread array of leds (no hot spots in a shallow tank), full sunrise/sunset ability, easy to use remote control and has both legs and a hanging kit. IMHO it's by far the best value in a full feature led fixture on the market.