Using the 2 400 watt lamps will provide more overall intensity but the center of the tank will receive less light as compared to the 3 250's. It is a tough choice between the 250's and the 400's. What is your overall goal witht the tank? What kind of corals do you plan to keep? Is this more of a display type reef or a propagation setup?
The 10K bulbs will tend to be yellow or crisp white while the 14K will be a bit bluer. Generally speaking the lower the Kelvin rating the higher the PAR level. (Photosynthetically Active Radiation)
Here are some links for comparison -
http://metricksystem.com/chris/250wcolour/
http://www.cnidarianreef.com/lamps.cfm
http://www.coralreefecosystems.com/ltg_bulb_color.htm
http://www.diyreef.com/lcomp.htm
Hopes this helps!