i got the 400w switchable because i can upgrade later. the 600 seems optimal but you have to start somewhere. i wasnt disatisfied with a 150w but you will want to get bigger when you can(almost guarenteed). get something that is sort of universal and you can use it in later projects. 400 will/should reach deeper and get bigger plants, but if you are doing a sog or something you might want a 250. but then you need 2 ballast to go with them. 6 of 1 half dozen of the other. lol.