[Sizzler]

Assuming both are aluminum blocks,
then it comes down to resonance and
structural integrity.

The small block is small. To get the bore necessary
for the given displacement, you're weakening the
cylinder sleeves more than you would with an FE.

The iron Genesis blocks, with no other changes, will show 50HP
more on a dyno than a stock 427 block. Why? Because the
cylinder walls are thicker, and siamesed. Why do racers
fill their blocks with concrete? Stiffness.

As mentioned above, the larger bore available with an FE
also allows for better breathing. The taller deck allows for
a better ring package and/or better piston where you
don't have rings squeezed together or too close to the chamber.

Then of course, the FE is crossbolted. Good for bottom end strength, and overall stiffness again. As for rotating assembly weight, send it to a good crank man and you'll be down to small block weight shortly. The lightest FE crank I've heard of was in
the 60lb range.