Interestingly enough, stinkybudz has a good point. Plants are quite sensitive to conditions that replicate seasonal changes in day length, and it stands to reason that light type and angle should play into that.
The clear blue spectrum light of a MH is what the plant needs when it is vegging. Lumens in the wrong spectrum are wasted lumens. You can get plenty of blue lumens from flourescents too, and if you plan to flower with a small HPS (under 600w), it makes more sense to veg under flouros, or a REALLY small MH (250w). Keep the equivalent watts ratio at 1:3 veg:flower.