Going with a roller cam setup in the 250 is going to add some big bucks to the price tag of your build. If you already have components for the 250 installation such as headers, intake manifold, this will make the 250 build a bit more attractive. While the 4200 will require the exhaust to be custom $$$$, the oil pan to allow an install into your vehicle($650-ish), either an adapter to install a conventional tranny in it or use the 4200 tranny which may require a custom driveshaft.
In the end, I think you will have considerably more HP with the 4200 on 15 lbs of boost than you will with the 250 with 15 lbs of boost. So if your goal is max HP, then the 4200 is going to be the best choice for your build.
One final thing to remember, with both of these engines having almost the same cubic inch, and with both of them bone stock, the 4200 has 150 more HP than the 250 does. So as long as you perform similar mods to each engine, the 4200 will always have at least a 150 HP lead over the 250.
