go with 600W you will want digital ballsts so that you can run both MH and HPS with the same ballast. Veg under one buld and flower under the other or run both at once with a seperate small room for mothers and clones under flouros.

You will wish for a 600 if you buy a 400...coming from experience.

And for gods sake (sarcastic) why does everyone want to change 18/6.....18/6 or 24/0 for veg and 12/12 for flower..

As for bulbs..I have great success with these:
Sunblaster T5 Fluorescents - Jons Plant Factory

Do NOT cheap out on Lights, Soil, Ventilation and Odour control.