HQI is available in 150, 250, and 400 watts. It is not available in 175 watt configuration. It is a little better circuit and usually yields a little more light output. As far as I know it is required for double ended bulbs.
I found 175W MH fine for a Softie/LPS tank. 250W gives you more bulb options as well as some additional suitability for corals. Do keep in mind that cost, power consumption, and heat output should be factored into your decision. Having to upgrade your household electrical service or implement a cooling system for your tank can be quite expensive afterthoughts.
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