What kind of dihedral would be most stable?
More dihedral doesn't necessarily make an airplane more stable. With too much of it, oscillations like the Dutch roll are not damped any more.

A polyhedral wing can be chosen for different reasons than just stability: With the dihedral of the outer panels, any angle of attack change there is reduced by the cosine of the dihedral angle. This means that the lift curve slope is flatter there and stall happens later. This is a great way to combine an almost elliptical circulation distribution with pleasant stall characteristics, similar to a raked wingtip, but using dihedral instead of sweep!

For the stability derivatives of partial dihedral I recommend to turn to no other source than Sighard Hörner's Fluid Dynamic Lift. In chapter XIV it shows a plot of a rectangular wing with an aspect ratio of 6 where the location of the onset of the dihedraled wing fraction is plotted over the X axis. This data is taken from NACA's technical report 548:



Figure 13 from Chapter XIV of Fluid Dynamic Lift by Sighard Hoerner. Note that it plots the derivative of a derivative on the Y axis!

The $c_{l_{\beta}}$ curve can best be approximated by a sine curve. Note that the Y axis shows the $c_{l_{\beta}}$ and $c_{n_{\beta}}$ derivatives over changes in the dihedral angle $\Gamma$. To arrive at your expression with constant tip elevation for different dihedral onset locations, the curves need to be modified by dividing them by the cosine of the partial span (assuming the root is 0 and the tip is $\frac{\pi}{2}$). This would yield a constant value for $c_{l_{\beta}}$, so the dihedral onset is of secondary concern and the dihedral effect depends on the tip elevation. Note: This is valid only for rectangular planforms - as soon as taper is involved, things get more complicated. This PDF has more on effective dihedral of tapered wings.

NACA TR 548 supports your opinion:


  The tests also showed that with dihedral only the outer one-fourth of
  each semispan, the dihedral effect was maintained to a much higher
  angle of attack than when the complete wing had dihedral.