If you have in issue with running costs, go with the CFL, you'll need at least 2. It all depends on the plants nutrient uptake and size of the grow area. The 100w HPS is more has more Lumens, which means more light. But it does cost more to run. The CFL is much more energy efficent but lacks the amount of lumens for vigorous growth. It all depends on the amount of space, plants, and nutrient uptake. If using a low wattage light setup, youll want to start at 1/4 strength nutes and work your way up. So as not to stress the lil ones.

Hope this helps.