This is a bit long winded, but hopefully answers your question...
So I researched this topic a whole bunch before purchasing my current setup. Basically, the DE bulbs are meant to be pushed by an HQI ballast. That being said, the produce more light, more heat, and typically more PAR (a rough measure of growth) at the cost of more electricity. As mentioned above, the pendent is the biggest determinant of efficiency and direction of the light....after all, if the bulb generates a ton of light but a whole bunch of it is wasted then it isnt really that beneficial. If you decide to go with a DE setup, then i would recommend the PFO mini pendents or the Reef Optix 3 pendents. Both are very good at directing light into the tank. In fact, a 250w DE setup with an HQI ballast and one of those reflectors generates about as much light and penetration into the tank as a 400w SE setup.
That being said, there are a few downsides. Because they direct light INTO the tank (almost straight down) bulbs do not mix very well - so typically that means all the bulbs need to be the same and you need to make sure the pendant is not too far from the edges of the tank (otherwise you get a little bit of lack of light at the lateral edges). Also, a DE setup uses more electricity (even if you compare 250w DE vs 250w SE). Lastly, if you could figure out a way to harvest all the light created by an SE bulb and direct it down into the tank, you would get similar numbers for penetration of the light, PAR etc at a lower electricity cost.
Enter the Luminarc pendents. They are set up for SE bulbs and do the best job of any pendent I am aware of at directing the light into the tank. They do this while covering a bit more area than the DE pendents. Thus, with the right pendents (Luminarc), you can get as much bang for the buck with an SE setup on an electronic/pulse ballast (less energy cost to run) as you would with a DE setup on an HQI ballast.
Above all, do not go with a cheap SE reflector...that is the worst option and in order to get the same amount of light as the Luminarc or a good DE pendent you would likely have to upsize from a 250w system to a 400w system - and that is costly in the long run (electricty).
I currently have a 250w DE Reef Optix 3 with HQI ballast and run 14w DE Phoenix bulbs (which I love). But if i had to do it again, I would go with an SE luminarc setup on a less costly to operate ballast. The DE bulbs and ballasts are more expensive than their SE counterparts...however the Luminarcs are expensive in the SE setup. The ballast in the SE setup makes up for some of this in that it is less costly to run and pulls less electricity.