Is there a big difference in using a 600w HPS vs a 1000w HPS in terms of plant growth and development?
The difference will be sqft coverage. You can do up to 5x5 with a 1000 watt hps. i wouldn't go over 4x4 with a 600 watt hps (3x4 would be better imo) so the difference is in sqft of your grow.