Personally, for a 72g I would recommend going with a Tek fixture. You can keep pretty much anything you want and you don't get the heat issues that you do with mH. Also you can tweak the color to be as purple, blue or white as you want using different bulb combos. If I was converting my 72 that is what I would put on it, and save the mH's for a bigger and deeper tank.
If you go with mH, 2 x150w would do, but you may find you have heat issues. Pendants that I thought were nice were the lumenmax, lumenarc, lumenbright (all lumen pendants tend to be rather large as a warning) and the reef optix III.
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