Well the EVO III spools fractionally slower and flows more, so yes if you want to go for 400 hp you'll stand a better chance with that over the Small 16G. That said whilst people have made 400 hp on an EVO III and over 350 hp on the Small 16G, it is neither the norm, nor easy and I'm not sure you could expect a normal life span out of the turbo if you intend to push it that hard.
Another thing to think about is that these turbos used to be cheap and a great bang for your buck. That is rarely the case anymore. I am also sure if Mitsubishi hadn't changed the engine orientation in the EVO IV that we would ALL be rocking late EVO twin scroll turbos by now.
If you want things simple then go for either of the above turbos aim for a little less horsepower (say 300-350 hp) and enjoy the car for what it is. If you really desire more power then why not consider getting some fab work done to allow you to use late model EVO turbos on the car? Yes it may cost you for a custom manifold and downpipe but the upside is that after that, stock EVO 8 and 9 turbos can be found for cheap far easier than EVO III ones, they are also twin scroll and I would argue can make more power.
I'm still exhausting my supply of standard configuration turbos but I do believe that within the next five years or so, the supply of these turbos is going to dry up and the price of the remaining ones will make it very uneconomical to stick with the 16G standard configuration setup. I truly believe the way forward is to drop some coin on a decent quality manifold and downpipe and start stockpiling a few newer generation 16G turbos whilst they are easy to obtain.
