600w HPS is a great light. works well for veg and flower. IMO MH is a waste of money as it puts out less light while using the same energy as an HPS setup.
600w HPS is far more efficient than 1000w lights especially since the 1000w lights have to be so far away from the plants to keep from "bleaching" the leaves or causing heat burn.

You may want to look at your nutrient regiment as the reason why your buds arent filling out. Or possibly the strain.
to increase yield try using a bloom feed with higher levels of P and K or using a bloom booster. I use bud blaster by supernatural. very concentrated and works well. you will notice yellowing of the leaves when using bloom boosters, just add nitrogen to your bloom feed to correct the nitrogen deficiency (yellowing leaves).
Another thing to consider is the use of simple carbohydrates like sucanat sugar or molasses. those are the cheap way to go but every bit as effective as products like sweet or carbo load. just add raw sugar or mollasses at a rate of 15mL per gallon on top of your bloom feed.
Avoid using simple carbohydrates if you get fungus gnats as it will make the problem worse.

so...600w HPS is my reccomendation. if you want to post your feed schedule or at least what you are using, I would be happy to give you some tips to dial in your nutes.

hope this helps...
GanjaRobPDX Reviewed by GanjaRobPDX on . MH or HPS to flower with? Which one do you use when flowering or which one is better? Also, Im about to start my 4th grow soon but have never really being happy with the thickness of the buds using a 600w HPS. Would a 1000w improve the thickness of the buds? Rating: 5