70 watt HPS or 150 watt HPS for cabinet grow

I'm designing a small box; dimensions are approx 20x20 square, 3 feet height. I'm debating about whether to mount a 70 watt fixture with a built in ballast or a 150 watt fixture with built in ballast. the 70 watt fixture is 9x9 inches square and the 150 watt is 12x12 inches square. The 70 watt fixture would allow me more room along the sides to mount fans to suck air from the bottom of the box to the top and out the back. The 150 watt fixture does not allow much room against the walls of the box to mount fans along the sides.

The box is not huge, only 20x20 square and 3 feet high... So, my question is would a 70 watt HPS @ 6000 lumens be enough light to flower a few small plants or one larger plant? I think it would be enough but I need someone elses input on this. I plan on using the HPS through the entire growth cycle from veg to flower. I know MH is reccomended during veg but HPS will also work. I'm buying the light soon so help is appreciated!

In a 20x18x8 growspace I found the 70w to be a little underpowered so I would suggest you go for the 150 as you have more space than I did.
If you look at the tech specs of the 70w they have an initial lumen output of 6000 but after 5 minutes this drops to around 4600. In your space this gives about 2000lumens/foot which is pretty low.
Look for a light with a removable gear tray so you can mount the ballast outside to help lower the temps.

i grew with a 70 w hps , just added cfls to bring up the lumens worked good so far

My old lowryder box was 20"x16"x32" and I used a 150hps which seemed just about right for that size.