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.