IMO, if you want to go with MH you should go with the 250W. You have so many more options. If it is too much light you can raise the light higher or put some screening over the top of you tank to shield it a bit. If you go with a lower watt light system and find that it is not enough light for some of the corals/inverts that you wish to keep you are pretty well stuck with an upgrade which would mean that you have to buy 2 light systems instead of one.
I have a 250W penant over a 24g tank and it is just fine. In the summer you can always set up a fan to blow across the surface of your tank or just adjust you photoperiod to have your lights come on during the cooler part of the day.
175W can be somewhat difficult to find and your bulb choices are limited. IMO 150W are not all that useful and I think you would be better off sticking with your T5's.
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