400 watts of cfls or 400 watts of HPS is still 400 watts being used. The big difference you will see is in the lumen out put. The hps lights will get you much more light for the same amount of electricity used. More light means more/bigger/denser buds.
As far as heat generated? I've so far only used cfls. Right now I have almost 450 watts of cfls, and if I'm not careful, my temps could be through the roof. Now that summer is over, the worst is over, but I'd like to soon get an hps so I can see firsthand the temperature differences.
And if I do get an hps, it'll be a 400 watter. Mainly for the available grow space I have, but also because it's close to the wattage I currently use with cfls.