I think either one would give you great results and if you can handle the heat issues...I say 600...but it will get hot, so a very good exhaust is mandatory...and if you decide on 400? you will still get great results...just not quite as much as a 600...but heat can make or break that bank. let us know what you decided and more about your set up?
If you have a couple of dollars to spend up front (you'll get it back first harvest) and you're not stuck on hps, I'd throw one of these 315w CMH lightsin there, they were made for that space. They'll yield somewhere between the 400 and 600 and you won't need anything more than 4" inlines to manage the environment..
I've got a 3x3x6 with a 600w. It gets a bit warm, mid 80's,but since adding a portable ac the Temps are now in the mid to low 70's. I've got a max fan pro and it is far from sounding like a jet liner
I have a 3x3x6 with a 400W and have 4 plants in there and there outgrowing the tent i honestly wouldnt suggest anything any bigger unless u really plan on pruning and working on your plants ALOT! lol. but the bigger the Watt the better outcome. my tent stays at 76 average with 2 16 Inch duct fans!