400W is 400W. Is the 200W CFL true or equiv watts?
Watts can be converted to BTUs.
Now that being said, yo can look at it this way. Say the HPS bulb temp is 200F, and the CFL (bulbs/internal ballast) runs at 150F.
The wattage will dictate how fast the room reaches the saturation point.
Being both rooms would be at 400W then should reach the 200F and 150F at the same time, everything else being equal. (the HPS would lag a bit since it has a little further to go)
Build and test. If you already have a HPS, test that and then test with the CFLs (they are cheap enough to test) If it winds up with a minimal diff, go HPS! The testing is paramount in getting the room temp right (with small rooms). Also keep in mind an empty room will cool easier (the plants will effect the flow through the room) so give it a few extra degrees as a buffer.