Conventional wisdom says halide for veg yes. I guess yes if you are blowing money on a dual ballast might as well use it but honestly I haven't seen any less growth using hps in veg but then again I just throw more light and don't give a rip about the power bill.
I'm the cheap kind of guy who would never buy a digital or dual purpose ballast to begin with. I would take that kind of cash and get a couple 600w mag ballast dedicated hps's ($120 or so ea) and DIY cooltube them and grow the fuck out of that scrog. Screw halide, if you throw 1200w of hps on that thing it's going to grow, like mad...
(of course you'll have more heat and power bill will be higher, but for the money and end result that would be my choice.)
But many people aren't DIY people and also if you have high energy costs...
Anyway my point is I always put the money into flower power (so to speak) Yes halide would be the most efficient for veg, if efficiency is more important that yield.
I should have explained that more, forgot to mention anything about getting more hps light.