No matter what bulbs you pick, you're only going to get decent light penetration 3 feet down... 2ft if they are very bushy and cramped indicas. So I'd say aim for 4ft tall plants maximum. If they are mostly sativas flower at 18 inches, if they are mostly indica flower at 24 inches. Taking into account pots and room between the canopy and lights, you shouldn't need any more than 7ft of space. Also, I'd suggest the 2 400watts. At the very least, it means if your ballast or bulb fuck up, you can grow with 1 light for a while until a replacement comes in and not worry about all your plants dieing. I'd rather pay for the extra wattage (and possibly have more/bigger buds anyway) and not run the risk of losing a harvest 5 weeks in because my lights shit the bed.
Also keep in mind when considering the height of the plants, keep in mind that the longer you wait to bud, the more room their roots take up and the closer to maxing out their size they are. I've had lots of plants that didn't grow buds nearly as large as they should've because I let them get too close to maxing out in size and root room before I flowered them. Make sure those things get lots of root room (if you intend to be growing 16 plants every square foot which is probably smartest if you want them to be 4ft, make sure they are in 3 or 5 gallon buckets... 5 gallons would be best).