Metal Halide is best for veg & High Pressure Sodium is best for flowering but both different types will work well for the entire grow,MH replicates closely the natural color of the sun in spring time with a bluish tint to the light & HPS closely replicates the color of the sun in late summer with a reddish tint to the light.
Unless you have a dual ballast & taking into consideration that your ballast is an HPS ballast just stick with the Hortilux bulbs,they kick ass,MH & HPS bulbs are only interchangable in dual ballasts so ya need to stick with the type your ballast is designed to run.
One thing ya might want to do is go to home depot & pick up a generic 400 watt bulb as a spare incase your good bulb blows,its rare they blow but it sucks running around trying to score a bulb when they blow.
