it seems to come down to depth- HQI bulbs have more 'punch' than SE bulbs, SE bulbs are known for better consistancy of color...
for example, I'm putting a 250w SE Radium (20k) bulb overdriven by a HQI PFO ballast over my 37g tank. the tank is fairly deep at 21.5", so I was worried that 150w wouldn't cut it, especially since I will be having the bulbs around 10" from the water (total 31.5"). In a custom made hood.
w/ HQI bulbs you can keep them closer to the water, due to the fact that you need to have a tempered glass shield between them and the water line (due to the fact that HQI bulbs aren't encassed in a UV resistant glass like SE bulbs), making splashes of water a non-issue... if you use SE bulbs you can't have glass between them and the water, since they already have a glass UV resistant sheild around them- any more glass will decrease the amount of light getting to the tank signifigantly. Hence the deeper you go, you will need a higher wattage SE bulb than HQI bulb.
If I were to use a HQI fixture that stood on legs above my tank, I would go w/ a 150w HQI fixture w/ two T-5 or PC bulbs for actinitic supplementation. Most fixtures keep the bulbs about 4" above the water line.
Since I am planning on a custom hood for my tank, I will be going w/ a 250w SE Radium bulb around 10-12" above the water w/o any supplementation from PCs or T-5s..you need to keep SE bulbs pretty far from the water line as any splashes of water will shatter the bulb when its on.
Different types of ballasts change the color of the bulbs as well- an Icecap electronic ballast makes a 20k bulb look much more blue and than an overdriven 20k bulb on a magnetic ballast, but w/ a life expectency of about 1/2... lots to read about!
Lots of links on this site if you do a search- leading to tons of links that compare the PAR of bulbs based on ballasts, etc...
Good Luck!