250w MH vs 24" ATI Sunpower 6-bulb?

Main difference would be light coverage. I think going the 8 bulb route would be better for coverage.

 

As a former ATI Powermodule user, both 6 and 10 bulb units, I can say that I am glad I went back to Metal Halide. I am running 400W XM 20K, but recently tried the 250W Radium, and fired with a true HQI ballast, the light is crisp white, with a hint of blue...it's nice. The shadowing, shimmer with MH is hard to beat, between the two lights. With the Powermodule, sure more of the coral is wrapped in light, but it doesn't look as natural as it does with mh. Even with the 10 bulb, I had trouble getting the right combo of bulbs to get the the look I wanted.

If you made a typo when you said a 30"x18"x18" instead meaning 30x30x18 for a cube, then a 250W setup like you are thinking about would be a good choice. I must say that I like the look of the radium fired with a true HQI ballast over the overdriven Galaxy ballast. They still make Hamilton and Aquamedic HQI ballasts, you may want to get one of those instead. They even have fans built into the ballast box.

A 10 bulb would be better for a 30" deep tank....my 40x28x22 was designed especially for my 10x39W Powermodule. No problems with growing coral anywhere, at least sps, but chalices and acans suffered if not shaded.

The ATI's light great front to back, it's the ends that don't give off a lot of PAR.