In your case, I would probably recommend power compacts. They will give you the highest output to cost. Next choice would be VHO followed by halides.
Actually, this 50 gallon isn't really the ideal tank for a reef (due to its non standard odd shape) Keep in mind that most lighting systems, sumps and stands are most commonly available for 55 gal or 125s. I think that you'll find most of the cost associated with a tank is *outside* of the tank (making the choice of tank most important) This is all covered in detail in my book but I would recommend the standard 55 gallon, next would be a 125. Everything evolves around the physical size of the tank. lighting etc. If you want some links to online retailers check out my links page Hope that helps some Robert