What are those again... 27" tall? If so, I still think you'd be fine with the 250w.So in the 300DD, what's your opinion - 400W or 250W? I personally plan on keeping my acros up higher and supplementing with blue LED's. I know 250W units would be easier on the utility bill. LPS on the bottom along with my ginormous 16" squamosa...
I can grow sps on the sand bed. Actually, some like my ORA bird of paradise HAVE to go down on the sand bed to do well.
You can also play with reflectors (if doing a retro, not a fixture)... some spread the light more and some focus the light more.