Like keeko said its going to come down to if you can keep the room cool or not. My guess is that the small space you have to work with would be difficult in keeping cool. I know you have a smaller HPS but still it will put out a good deal of heat which will eventually affect your yields.

Another factor to consider is that an HPS light will raise the bill a little bit (in your case probably no more than $20 monthly), and also considerable space. Not to mention, there is usually an attached ballast on those smaller units (and some larger) which adds to the overall weight, - got anywhere to hang it from in that closet that can support 15+ lbs?

I guess if you really wanted to have 2 flowering plants and then have your CFL's used for vegging / cloning then that would be a good way to go and that is what I would do personally. HOWEVER, you are limited on space so I would suggest adding more cfl's thus keeping electricity costs low, heat low, space maximized and costs DOWN! Course, if you want those lower bud sites to get plenty of light there is no avoiding an HPS light.
deftdrummer Reviewed by deftdrummer on . Lighting Question! Is it worth it? Ok I found a website that sells a sun system 4 150 watt HPS light for like $100 brand new with a 5 year warranty and as of now Im growing with three 27 watt CFL (105 watt output) but for one plant. My environment for flowering is about 3ft wide x 1.5ft deep x 3 ft tall. I currently am pulling decent yield, but I wanted to expand to growing 2 plants at once. Will this single HPS lamp be a good buy and will I get better yields over the two plants with this light as opposed to the one with the Rating: 5