Well I honestly don't know much, but from what I have learned I can tell you this. CFL lights encourage flowering. If you have autos then you shouldn't have to worry about making them flower. HPS lights gives more energy so they will help your plant grow better. But HPS lights also get hot. So if your cab is too small it's probably your best bet to use CFL lights or else you could risk burning up your plants! Hope this helps! ~Higgy
75w is pretty low for an HPS, will have same lumen output as a few 60w CFLs but with way more heat. I have a 150w HPS that does 15,000 lumens and it worked great for a 2x2x4 grow box with few plants in it but now that I have a 1400w 200,000 lumen setup I wish I hadnt wasted the money on the 150w. The 250w cfl will provide a better spectrum for all around plant growth, where as HPS are better for flowering.