I've used both and I like both, back when I used DE's the best bulb you could get were 10K's, now we have the same choices (almost) in both DE and SE..I've been running 400 watt SE's for the past 4-5 years, I'm going to soon be switching to 250 watt DE's..
Tell us about your tank.
You can get a 250 watt HQI ballast and run both if you can't decide.