Now when you say this room is Air Conditioned. Is this grow room airconditioned or is this like in a closet in an air conditioned room?
If you are going to use all of the power to run the A/C you don't want to run a 400w. You could run a 1000w in an a/ced room no problem. You definitely don't want to run a 400w.
If it's a portable A/C (and a smaller one at that) it will run about 800w.
400w = ~50,000 lumens = 1200w with A/C
600w = ~96,000 lumens = 1400w with A/C
1000w = 150,000 lumens - 1800w with A/C
Maybe you don't know the difference, but I have found that average yeild to break down like:
400w = 4-5 oz's
600w = 8 - 10 oz's
1000w = 14 - 16 oz's.
Now those are rough estimates it's easy to underperform and very possible to overperform those numbers, but those would be good average ranges.
You can grow almost TWICE as much bud in the same space for 200 more watts. The difference between 1200w and 1400w.
It doesn't make any practical sense in that scenario to run the 400w.
The 600w is the most efficient and the best bang for the buck I'd say and the 1000 would be maximizing your yeild in your space.
You're going to need to ventilate the room the same either way and all else equal, you will get a LOT more going to 600w.
I'm running a 400w right now while it's summer time and I'll run 600w or 1000w in the winter because the difference right now for me is 400w without an A/C or 600w with an A/C. That makes it really a 1400w or a 400w and I'd rather just take the lower yeilds in the summer and larger in the winter. Thereby maximizing my energy efficiency.