Echoing what wasserpest said, you would not have the spread of your current system. You could look at two 150 watt fixtures instead. Not sure they would grow things any better but might help punch light down to the substrate if you are growing a carpet of HC for instance
My 90 had two 150MH when I bought it. Growth in the tank before I got it was slow to nonexistent. I think the pendants might have been too high though. I didn't want the MH and went with T5 instead and growth is out of control. lol
With the T5 I have more control of the looks of the tank by combining bulb color temps. I can also run half the bulbs for for 3-4 hours and that helps control the operational cost. It has 2 pair 54w.
The only positive I see for MH is the glitter lines you can't get with T5 or PC. IMO MH are energy hogs that often draw substantially more power than the quoted wattage depending on your ballast and bulb combination. I'm no longer willing to pay that price every month for glitter lines.
you cant never go wrong with T5, enegy saver ,less heat compare to MH , but again that's not the main of the topic. if i gotta to choose between the PC and MH . i'll just do Pc like steve said on the post above.