600 hps vs 400 mh for veg

So I'm currently vegging under my 400 watt MH but i now have a free 600 watt hps available, would it be better to veg with the 600 hps to speed things up?

I only plan on vegging for another week and a half.

Edit: Also can anyone tell me if a stronger light source helps roots better develop?

Depends on your plants. If the 400w is enough to give them all light without the plants furthest away stretching, then there's no need for the 600. The extra time it would take a plant under a 400 to reach the height of the same plant grown under a 600w for ten days could be counted in hours.

Stronger light increases plant growth up to the maximum light the plant can handle, if the plant has the nutes for the faster growth. This includes the roots but root growth is just as affected by things like moisture levels as light levels.