If your not running into heat issues, a 1000 is going to produce more bigger buds. 2 600's are just do not have the intensity of a 1000. The greater the intensity, the more and the bigger the buds.
If your doing a small grow and keeping it simple, sure the 600 is a fine little bulb. If your trying to max you yields, then 1000's are the way to go, perferably verticle.
Different typs of grows are going to be more advantageous to one or the other. If your a scrogger, then perhaps a million 600's is better since you need little penetration and are basically top farming. If your doing taller stuff then verticle 1000 tree farming will blow scrogging out of the water on most levels if not all.
Lots of ways to skin a cat, but realize in optimal conditions your 1000 buds are going to be a bit bigger than the 600. Im sure there is some exception like 8 600's around a plant instead of 4 1000's. Being able to stack lights vert would make for huge yields but thats not very praticle beyond 4 plants.. The 1000 might still outyield it because of the better penetration, so its hard to say, but as the buds from a 600 pale to buds from a 1000, buds from a 1500 are even nicer!