Isn't weightlessness the obvious choice?

I could easily believe, and will accept as likely until
given reason to do otherwise, that a person raised in
zero-G will have a different synaptic wiring than a person
who is gravity-bound his entire life.

Would this person have more complex wiring than an Earth-
bound person, an "Earther?" I'm not so sure. This
"Spacer" would not have any synaptic wiring for keeping
balance, for those dozen or more adjustments per second
that Earthers have to make, just to walk. He might have
more complex wiring, or may not, but I'd guess it wouldn't
be that impressive. The Spacer is adapted to one value of
gravity (none), just as the Earther is adapted to one
value of gravity (9.8 m/s^2).

But Spacers would be more limited than Earthers. Earthers
CAN adapt to zero-G, at least somewhat. This Spacer would
have a hard time adapting to Earth, or even to Mars or the
Moon.

If you really want some complex movement adaptation, what
you want is *variable* gravity. You want your Spacer to
be comfortable, from birth, in 1 G, 0.5 G, 0.1 G, or
zero-G. This guy has the synapses for walking or running
on Earth, bounding on the Moon, or floating freely in
weightlessness. More conditions, more adaptation, more
complex wiring.

Now, I'm not convinced that this means that he's *smarter,*
but if that's what you're trying to do, then variety is
the spice of life. And it's in the classic O'Neill habitat
that you find it.