Well that depends on your growspace, and how many plants you want to grow in it. Any larger than 4' x 4' and I would go with a 1000W. It's only an extra $50 - $100 for the 1000W. I'm not sure, but I think you can also get a 1000W ballast and reflector and run a 400W bulb in it if you want that security in knowing you have extra power if needed. The air-cooled reflectore are ALWAYS a great idea and I would pay extra for it anytrime. Hell, I run an exhaust fan for my 250W HPS reflector. Anything I can do to keep my girls happy and healthy.