If this is a standard 90g with the usual center brace then I think it's a no-brainer - go with at least 2x250W MH and consider T5 supplementation (and preferably use HQI/DE on a non-electronic ballast for the best PAR). If it's going to have eurobracing then you
could get away with a 400W with a GOOD reflector (something like a Lumenarc). But even then it's going to lead to a sort of patchy spotlight effect with major dimming on the ends of the tank (which may be a good thing if you're planning on keeping low light corals - you'll have somewhere to put them).
If this is going to be an SPS dominated tank then I think you need to go here:
http://www.advancedaquarist.com/2006/6/aafeature2 and read all 4 (I think there are 4) parts of this series. Too many hobbyists get brain washed and hung up on the idea of running 400W MH and put far too little consideration into planning
appropriate waterflow for SPS tanks. They then become frustrated when their SPS lack color and growth despite having "super bright" lights and having tried every brand of blub in every available kelvin rating.