324w used for a 6 bulb T5 compared to a minimum 500w MH without supplimentation is really the big difference. Make sure you put your lighting 10" above the tank if you decide to go with MH and don't forget to get 2 4.5" fans for the canopy if using MH. The MH will heat up the tank a little bit, I keep my house at 73 to 74 degrees and if the house get's above that the temp in my tank jumps 2 degrees where it is already running at 81 to 83 degrees. There are pros and cons for both, being that the T5's are only good for tanks up to 24" in height, which is right where your at with the 90g.
This really has to be your decision, to weigh the pros and cons as to what you want to do with your tank.
There were two reasons I did not go with the T5's 6 bulb setup.
1) They sit 2" from the top of the tank and with me leaving a lot for Training and such, I needed to make the tank very accesible for feeding and cleaning the glass for my wife. My new hood with the MH has feeding doors in front and I can pretty much get in the tank and do what ever I need to do without taking off the canopy. I could not do that with the T5's.
2)I wanted to be able to do anything including crocea clams and be able to put them anywhere in the tank as well as seeing the shimmering effect to see if I like it. It makes my wife a little dizzy looking at the tank for any long period of time, with that shimmering.
Lighting is a tough decision, here is the latest picture comparison from my old T5 to the new MH, this is my X-Mas picture of the tank and a small video to see the shimmering effect.
http://rjwilson41.com/docs/aquarium/90g/movies/rjs90gtank122505.wmv
T5 lighting before switching to MH
Latest picture under my 2x250w 14k MH with no supplimentation.