If T5 and MH are the only options your considering, then I would go with T5......at 18" deep 8 bulbs of T5 will give you enough light to do whatever you want as far as corals go and less issues with heat..........and more options for color........only downfall is the $200 bulb swaps
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260g mixed reef, 105g sump, water blaster 7000 return, Bubble King SM 300 skimmer, Aqua Controller Jr, 4 radions, 3 Tunze 6055s,1 tunze 6065, 2 Vortech MP40s, Vortech MP20, Tunze ATO, GHL SA2 doser, 2 TLF reactors (1 carbon, 1 rowa).
http://www.canreef.com/vbulletin/showthread.php?t=50034 . Tank Video here
http://www.vimeo.com/2304609 and here
http://www.vimeo.com/16591694