Until recently there was no such animal as a de 600 but there are folks like MWD and capulator that are huge proponents of the SE 600's,the de600/750 is so new i dont think there are any reports out so far.
They will be offering a 6/750DE. It is not available until September. I'm testing 4 of them now.
I've read the DE's are more efficient at striking the bulb.
The SE's I assume project light forward from the base of the bulb like most other SE bulbs. Could imagine they'd work great in a vertical application.
All three lights have overlapping wattage selection to dial in what you want. But from all that I understand over and under volting decreases useful life. Maintenance cost for all three are roughly the same.