Wattage is for the most part wattage. Using 10 40watt cfls is going to be almost identical to the draw off of a 400 watt HID.
The reason I say almost identical is due to the fact that some energy is wasted at the ballast. The true indication of the power draw of something is its amp draw. You can very easily find the amp draw for a ballast on an HID but to find it on a cfl, your on your own. Also, keep in mind that cfl's also use ballasts, only they are attached to the bulb itself.
Also, HID lighting has the option of running off of 240 volt current which is going to guarantee that you don't draw more than the amp draw states.
When you run off of 120 volt, you could in theory be drawing double the current due to how alternating current works. You have 2 different power sources entering your home and they both have to use the same amount of power. It's how alternating current works. If you draw 2000 watts off of one of the phases and only 500 watts off of the other, the phase only using 500 watts has to ground out 1500 watts of power which just gets wasted in order to balance out with the other.
Now, when an electrician first installed everything in your house, he also tried to balance out the load in between the 2 phases to reduce waste like this from occuring, but, when we as growers begin doing things that weren't accounted for in terms of electric draw, it is very common for us to unbalance that load and the end result is wasted energy.
So, I guess to sum up, if you're going to be getting 400 watts worth of lighting and you're going to be running it off of normal 120 volt, it doesn't matter if you go with a bunch of cfl's or if you go with HID as far as your electric bill is concerned, but if you get the hid, it will require less work for you and your plants will appreciate it's intensity more.
Happy growing