One of my tank is 2 ft 3 inches in ht and 6 ft long .... I use 2 x 400 w Aqualine 13000 SE bulbs ... it really depends on how you intend to place your corals .... for example, I have the sps that are supposedly exposed to extremely strong light right in the centre of the light .... there is a period of 'burning' even for these .... however, they return to vivid colors after 3 mths or so (this I observed from some of the acros I have) ...
For my faviites, blastos, mushrooms ... I place them semi-shaded ... and that creates some interesting possibilities ...
For example, my cynarina that is only about 3 inches diameter extends to some 7 inches under the partial shade ....
With my 150w and even my 250w MH tanks, when I attempt to execute such a scaping, the results are never as good as under 400w MH .....
Even softies like colt corals and gorgonians that generally look brown are lighter in shade and have better polyp extension directly under 400w .....
In sum, I feel that a 400w provides a wider range of aqua-scaping possibilities vis a vis 250w.