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It depends on what you want to keep. If you are interested in corals, clams or high-light anemones spring for the MH. If you are just planning on simple soft corals CF will be just fine. Look at the Coralife Aqualight Pro MH fixture as a suggestion.
Not really enough light for a sebea. Light penetration will be the biggest issue, especially in a 90gal. They tend to like to be at the bottom of the tank where the substrate meets the rocks. CF light will not reach it very well.
Do not worry about watts/gal, it is a bad bad rule. As I mentioned, penetration is what matters. MH create a much more intense light from a single point so it will reach the bottom of the tank much better.