how many plants you got?def the bigger one would be better if ya got the room.. make sure you get good ventalation with it also. i have a tent thats 4x4x7 with only a 150w HPS n 8 cfls for side lighting, works out great but i do need a bigger light SOON....
you got a link to your set up? is it in the journals?Either one will work for a 600w, assuming you have decent ventilation and an air cooled hood. Take a look at my setup...I've got the 57x57x78 with a single 600w and there is tons of additional room. I agree with smokintreez...you could get away with more in the larger tent. To be honest, I kind of wish I had gone with a 1000w but I may end up with another 600w in there in the future.