Your grow room square footage is a little over 3'6", 48"x20". The last measurement is height and only needed for cubic feet measurement. This is important when deciding on intake/exhaust fans sizes. You want to give your plants between 30-50 lumens per sg ft so lets go with 50 lumens x's 3.6' =180wts. Either lamps would work but you would get better coverage with the dual 150's as was mentioned, hope this helps...
Malcolm...