Get one of those new bulbs that does both.
600w HPS & 400w MH In One Bulb
The PlantMax Dual Arc Bulb incorporates the best of HPS and MH in one bulb with two simultaneously operating arc tubes that are optimized for peak performance. The 1000 Watt Dual Arc Tube Lamp runs a 400w Metal Halide and 600w HPS in one bulb providing full spectrum lighting for your plants. This PlantMax grow bulb runs on a 1000w HPS ballast and emits 110,000 Lumens.
USHIO HiLUX GRO Dual 1000W MH / HPS Hybrid Bulb
Covers the entire photosynthesis spectrum of your plant
For maximum branching, vegetative growth and maximum flower yield
The ultimate full spectrum 1000 watt lamp that covers the entire photosynthesis spectrum of your plant. For maximum branching, vegetative growth and maximum flower yield!