The Eye Hortilux 600 Super HPS is the best 600 watt bulb on the market, IMO. (you didn't say if you are getting a straight HPS or switchable ballast)

Whatever you buy, the ANSI code for 600 HPS is S106. Any quality bulb will have that code # on the package.

ps...eye doesn't make a 600 MH....go figure.