Growth is dependent on several factors which lighting is one of them. Water quality on the other hand is also quite important. In fact, it's the most important! IMO, if you can keep the water clear and nutrient free, you do not necessary keep those not-light-demanding spses such as monti on the surface. They can be placed somewhat near the mid section of the tank to provide another dimension to your tank because the light penetration can go deeper and further even with a 150W mh. There's this RC tank of the month 2 months ago (if I am not wrong) which was running on 150w mh supplementing with T5s and his SPSes are BOOMZ! Also, there's this reefer in RC called Robertus whose old tank used to be running on several 150w MHs. His tank was BOOMZ too! :D So yeap (:
150 or 250W, also look into your pocket and your long term commitment into this hobby before deciding. Electricity bills can be a killer when running on MHs.
Just my 2 cents.
Happy Reefing,
Marc J.