I just got an email back from Schmidt and this is what they said

>I have noticed that all the SON28 hubs have strain-hardened
> stainless steel axles, except the center lock hubs they have
> aluminium 7075 T6 axles.
>
> Which of the two hubs have a stronger axle?

The stainless steel axle definitely has the better corrosion resistance.
If you will use the wheel also in winter on salted roads or near the sea
with sea salt in the air (and the rain), we would recommend our hubs with
the full stainless steel axle.

Our hubs with the stainless steel axles have also a wider distance of the
bearings, which results in a little stiffer wheel - which is relevant for
heavy riders, cargo bikes, tandems or harder offroad use.

> Do you use the same size bearings in both axles?

Yes.

> And why doesnt the center lock hub have the same steel axle as the rest of the SON28 hubs?

It gives you the choice between a lighter more compact design hub and a
heavier, wider, more robust variant.