You can veg or flower with either one and the plants will do their job. HPS and MH just put out different spectrums of light (you will notice one has a yellow/orange tint, like the street lights, and the other is just a very bright light) that are MORE beneficial/useful at particular stages of growth but either can be used during the entire grow.
Kinda like a race car, take NASCrAp for example; same cars run oval tracks that run (real) road courses, they just set up the car differently for these two types of tracks. You can run the same car, engine, driver, ect on either track but they will alway set up the suspension and tires differently at each type of track to perform at the cars best. The car could run the oval suspension on a road course, but it just isn't gonna be at it's best, the driver can make up for some of this with his skill and may even out perform some of the other drivers in cars with correct suspension, but he could have done better if he was set up better, you know?
Hope this makes sense.
