I think that a 90g would be ok with two 14k 150w metal halides, as long as the bulbs are Radiums. If you want to go with more, go with two 250w metal halides with Radium bulbs. If you are looking for high light coral and inverts, you should probably think about supplementing with T5s.
I know a website right now that has a "convertible" metal halide ballast (150w to 250w) for $113.95) The Radium bulbs would be about $80 each. Of course, you would need one more reflector and socket set - should only be $50 or so for that, depending on the type of reflector you get.
A lot of the ability of a metal halide has to do with two things: The quality and spectrum of the bulb you are using and the reflector you are using the bulb with.
I really like T5's, I think that they are pretty awesome little light fixtures. I use one myself! Although, I am starting to realize that T5 bulbs are the same as metal halide bulbs in terms of quality. Stock bulbs on T5 fixtures are good, but they aren't great. If you really want to get a full spectrum and quality T5 bulb, your going to look at spending $25 to $30 a bulb. Either way, MH or T5, you are going to be spending $$ for quality.