Personally, I would do it. There is a reason that just about everyone does it and it is generally agreed upon that MH is better for veg while HPS is better for flowering. GG is right about the trade-off, as HPS are more economical giving you more actual light intensity per watt, however since the blue spectrum is what the plants technically "prefer," regardless of any individuals performance with any light source (any plant will grow under a 1000w HPS), I would veg with MH if you have the means. Actually, especially since you are planning on vegging for the shorter time period. If time were not an issue it wouldn't be as big of a deal to veg with HPS, but you will get better growth patterns (smaller internode length, etc.) with the MH in your couple weeks.