Hey,
MH bulbs can burn for many, many, many, many years... Well, maby not that long, but for a good while any-hoo. You should change them "every 12 - 18 months" because their spectrum starts to shift down. IE they become more yellow. As far as brand goes, yes there does seem to be a difference in my experience. I've used Hamilton, Sun Aquatics, XM and Radium. From what I've seen, you pay for what you get. The Radiums held their color for longer, and burnt brighter. Sun Aquatics bulbs are great - for a sump. Hamilton and XM seem to be a good middle of the road bulb - XM (I feel anyway) is the better of the two brands though. Keep in mind though, you need a pulse ballast to run Radiums.
Hope this helps!
*This is all just my opinion - so please don't pick on me!
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Have a good one!
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