Jim Laroy has also said the 906 was "a little" better than the rest. He stressed "a little" if I remember correctly. In other words, don't "just look" for the 906 if "ya gotta" have iron heads. Any of them will do.
The big thing with the 906's, to me, that is a bigger point of debate, is open vs. closed chamber. That's a bigger topic than the smaller differences on flow.
Dome vs. flat top pistons. combustion chamber efficiency (and everything encompassed by that)
The trend seems to give the nod to closed chamber heads
