[Delphinus]

The rule of thumb for changing them out at regular intervals doesn't really matter if it's SE or DE. In general though I've found that the higher the kelvin the sooner the shift.

As for the question of whether brand matters... I'm afraid that's really a question you need to answer for yourself with some experimentation over time. In general though, you get what you pay for. Having said that, there are a number of new bulbs on the market these days that are "economical" in comparisons to others and they seem to be just fine.

The one thing that I recommend though is trying to match bulbs to ballasts. This can help narrow down the choices when trying to decide what to try next. I.e., a manufacturer may put in their specs that a lamp should be run on a pulse-start or a probe-start ballast. Not that vice-versa can't work but sometimes you get sub-optimal results. I.e., if you try to fire an Ushio on a probe start ballast it may take up to 30 minutes after powering on for that lamp to actually light and stay lit. (After a while it gets better.. I ran Ushio's on probe starts for many years but if I could go back and do it again I'd have bought pulse-start ballasts... I bought the ballasts before understanding any of this kind of stuff.) Now that Iwasaki has a 175W 14000K lamp I'll probably never use an Ushio on my one remaining probe start 175W ballast -- I sold off the others. Great lamp, awesome colour, super bright, and intended to be run on M57 (probe start) ballasts. Unfortunately a bit pricey but with the 6500K Iwasaki's tending to be good for up to 24 months I'm hoping some of that Iwasaki quality spills over into this new line as well.

BTW Iwasaki's (6500k's) tend to have the best bang for the buck as far as PAR (photosynthetically active radiation) is concerned. If you want wicked coral growth these lamps are hard to beat. But a lot of people find them aesthetically unpleasingly yellow requiring a lot of actinic to balance out and make white. I generally wasn't unhappy with the colours after using them for a while (in fact that was the only period I really had nice SPS was when I used those lamps, but I shouldn't really mention that as there were other factors involved..) I switched to 10k's last year as I wanted to try a bluer look to see how I would like it. Also generally speaking the bluer the bulb the more colour you get out of corals.

Also one last thought, electronic ballasts (a third option over probe-start and pulse-start). Generally speaking if you have electronic ballasts then you don't need to concern yourself about lamp selection since they will usually fire any kind of lamp (of the correct wattage). Only thing is, they tend to underdrive the lamps slightly. Some people claim to notice the lights are less bright (I can't say for myself, I've never tried electronic ballasts .. I did try one, but I broke it before I ever got to use it.. $150 paperweight.. more on this later). But it does result in longer lamp life, which is a good thing. It also probably means your power bill is few cents less than it would be otherwise. And they say there's less heat from the ballast. The only drawbacks I see to electronic ballasts are 1) the cost, they're more expensive and 2) depending on the brand, if you break them (i.e., there's a short in the wiring somewhere), they're dead. Icecap ballasts can supposedly be sent in to the factory and get repaired, but other companies don't do such things. The one electronic ballast I tried was a circuit board encased in epoxy. There's no way to fix such a beast, so I had to trash can it. What happened was I was trying to DIY a DE fixture, and accidentally contacted one of my lamp leads to ground (I thought I was being cautious by grounding the fixture.. I read afterwards the warning "Do not contact lamp leads to ground". So much for that.)

Anyhow hope some of this info helps.

PS/Edit. One last thought (I realize I said "one last thought" already so this is "and yet another one last thought"), there are also "HQI" ballasts but that's a whole other discussion (they are still considered pulse-starts generally). 150W and 250W HQI ballasts exist which tend to overdrive the lamps slightly, giving a brighter look overall. Despite what you may read, there's no such thing as a 400W HQI. PFO markets a ballast labelled as such, but it is a really a 430W SonAgro ballast (actually intended for Sodium lamps, not Metal Halide -- but will run pulse-start MH lamps just fine -- just overdriven slightly).