I say yeah go for the 1000...really why mess around with the 600's when 1000w is usually cheaper - Longterm 600 is going to be cheaper cause less electrcity - sure 600W covers more area but its less intensity and less lumens and adding more lights doesnt really fix that - it just covers more area with the same ammount of lumens (a lot of people don't understand this part), so I battled this question in my head for along time 600vs1000 and I decided not to waste my time with the 600's and just get 1000's its cheaper start up cost anyway and electric cost is not an issue for me..600's is more efficent when it comes to coverage area and longterm cost, 1000W is more lumens and roughly the same or cheaper startup cost with a more expensive longterm cost to run the lights.. So the choice I guees is efficency in power and coverage area vs efficency in light penatration or lumens...
Basically: I would use 600's if I had to use a lot of lights because of a large grow room and 1,000 If I did not need many lights (medium-small growroom)..
Hope that helped....