HQI lamps require a protective lense and SE lamps do not. Coral Magazine had a great articile on this. I switched from DE to SE when my DE lamps where losing 50% at 8 months. I have had 12K Reef flux SE last up to 18 months with only 10% drop off in lighting. Some swear by them(DE) I know on my bosses tank which has Giesemen lamps they last almost 2 years. I think the trick here once again is to match the lamp to the ballast.