A 30g tank will be HEAVY, so I would make sure your desk can handle the weight first.
I would go DE MH
The DE are supposed to put out more light than the same wattage SE(single ended)
So with the 150w DE it is really about a 175 SE.
I have heard the 20k bulbs don't last as long as say a 14k or 10k, 10k lasting the longest.
Also 20k don't put out as much PAR as a 14k,10k, the 10k would put out the most depending on brand.
If you want more blue I would go with a 14k mostly for better PAR and they last a little longer.
I'm looking for a couple 250w DE pendents right now so just do some shoping around, there are good deals out there.