Imho, I would use metal halides to veg and switch to hps 10 days or so after they show sex. That is if you are running 600 watts.
If you are running 400 and less, I would advise you to use a Ceramic Metal Halide bulb from the start and never switch it. It simply has more red than MH and more blue than HPS.
Do the research and you will see it is the only bulb you need, and it works on an HPS magnetic coil ballast only.
The only drawback is that most digital ballasts destroy it, so youre stuck to analog ballasts, and it only comes in two sizes (atleast the philips mastercolor retro hps) 250w and 400w.
Its quite cooler and keeps most of the heat around the bulb instead of "throwing" it down towards the plants.