Unless there is a sudden increase in module efficiency, I personally don't see the point of 400w panels - at least not for domestic use. A 400w panel would be pretty much twice the size (and weight) of a 200w panel. So if you could only fit 8 200w panels on a roof, then you'd only get 4 400w panels.
4 400w panels may be a little cheaper than 8 200w panels, and may save on the odd clamp or wire here or there, but on the minus side, the 400w panels would be heavier and bulkier to get onto the roof and would allow less flexibility when arranging around obstacles or awkward shaped roofs.
In terms of getting the maximum power for a given area, it is really the efficiency figures you should look at. The leaders in the field (who tend to leap frog each other) are Sanyo (soon to be panasonic brands) and sunpower (currently at 19% and 20% efficiency respectively).