forged internals are stronger and will last much longer than any stock product from ford. High horse power applications put a lot of stress on the internals of the engine, and the forged internals are made to take this abuse. another plus to forged internals is that you can increase the stroke of your engine and in turn have higher displacement. I dont know everything about this, im sure some one else can pick up where i left off.
About the 5.0 question, the 5.0 was a pushrod motor, and as you probably know the 4.6s are OHC engines. This is much more efficient than pushrod. why they decresed the displacement i cannot tell you, but it is probably for efficientcy. But the 4.6 does make more hp stock vs. stock than the 5.0 so that sould have been in consideration also.