nothing real big different...there are 2 types of metal halides....1)mogul based which is has the single end like most light bulbs....2) hqi which are double ended and are "supposed" to run slightly cooler and last a little longer...they are smaller and more compact...the effect is the same for both just personal peference...oh and hqi's run off a different type of "lighting compound" opposed to the mogul based....i like my hqi's but i like the color of the tanks at LFS's when i go to them and they are almost always mogul based...that is about all i know maybe someone else can chime in