Power compacts are also on the way out, as T5HO are cheaper to run and much more effective. T5s are close to MH but will not give the shimmer effect and not as effective on tanks deeper than 18 ".
T5's however run much cooler tham MH which to me is the way to go, especially if your tank has a canopy. Without a good chiller I would not attempt to use MH lamps.
For a 72" long tank a T5 fixture will cost you about $600-$700 where as MH will be $1000.oo and up