DE bulbs are reputed to last significantly longer than mogule base bulbs when run on an electronic ballast. That's one of their selling points. If you had one go bad after six months, I would guess you either had a bad bulb or it's mismatched to the ballast you were using. HQI is a type of ballast, not bulb, and you can use an HQI ballast on either mogule or DE bulbs. In the case of DE, they typically need either an HQI or electronic ballast to fire. HQI ballasts drive the bulbs a little harder, which increases the light output, but possibly (I've heard stories both ways) shortens the bulb life.