MH is preferable to HPS for vegetative growth. Don't get me wrong, HPS works fine (I use it all the way through my grow) but it will slightly stretch your plant. As for 24/0 vs 18/6 is one of the most debated questions on this forum. Most cannot see the difference in growth rate between the two, so I'd opt for 18/6 for electricity saving reasons. Either will do fine though.