I'm into lightweight, so like the highest loft down available —800-900 down. Alternatively, this attitude makes no sense at all when you consider that I'm more than a little overweight, and so losing weight will have far more effect than lightening my gear. However, I can afford lightweight gear, but haven't been able to discipline myself to the point where I can get my weight back to where it used to be. (My birthday is today—I'm 58.)
Anyway, lots of opinions on this subject of down loft and fill power. The science on this subject seems to be sorely lacking.