This question needs more information to answer. What type of grow? Hydro, dirt, SCROG, trees? Vert or flat? One large room or multiple small ones?
Overall, 600's are more efficient at using electricity. They also will give you better coverage in a flat garden. The trade off is less intensity, i.e., less penetration into the canopy. Not so much an issue in vert.
Give us some insight into your layout, maybe we can help.
Fred