You can't go wrong with a 4 inch inline fan (~$70) and a variable fan speed controller (~$20). If you setup your grow space and trial run the lights for a few hours without any fans on and see that the temps are super hot and/or you'd like to also use a carbon filter to regulate odor (most likely at least one or both of these), I'd go with the 4 inch inline setup. If odor and heat aren't issues and you're running LEDs, you might be able to get by with a powerful duct fan (~$35) exhausting the warm air up top.