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[Aquattro]

What 400w will give over the 250 is a wider range of depth in which to place corals. While some people have 250w and successfully keep sps (with coloration), it is often within the top 6" of the tank. 400w will allow you to utilize more of your depth. The trade off is probably more heat, but with every pro there is a con. 250w HQI will give a comparable, if not better, range (for a lot moe money).

Again, just an example of the different opinions you'll get on lighting.