I have a Sunlight 600w HPS + 400w MH under one reflector. The ballast electronics are in one metal box. I like the idea of a 'blended' light spectrum but honestly there is no big difference in growth/yield (that I can see) when compared to say running a 1000w HPS.
To answer your question : If you're also intending to veg. then go for the 'blended' system; if only for flowering then go for the 600w HPS.