My experience with 400W HQI is disappointing. I am running three 400W PFO pendants at this time and have had higher than expected failure rates with arching at the connect which wipes out the bulb. After the first bulb, installing new bulbs are most difficult. I have not had this problem with 250W HQI for which I run 4 Sunlight supply pendants. Bulbs selection is also very limited for the 400W HQI, with little competition, thus the prices are higher than moguls.
As for the xm 20Ks in 250W, they are not very bright, so I am going away from them. They do last long, but reeflux and others are much brighter in my experience.